I suggest you do the following: click tools>preferences>general and set a minimum font size and also untick the box to allow other fonts. At top of than pane will be likely shown Latin. Repeat the process exactly for Other Writing Systems from the drop-down menu. Leave tools>preferences>Composition to 'Variable Width' and I think this will resolve your concerns. Also, you might be happier to turn HTML off, as you have more control of fonts and size when using plain text only.

Thank you for your thoughtful suggestion.

I may be misunderstanding, but it appears that changing the "Fonts and Encodings" is not something that I can do while I am creating a reply to an E-mail. I also do not understand how this suggestion would address the issue that I raised which is that when I explicitly request Thunderbird to change the font on my reply it ignores the request and leaves the reply in the unsupported font. If Thunderbird is going to put its implementation ahead of the interests of the user then I would appreciate it if Thunderbird would explain why it is rejecting a request which seems reasonable to me. Modern software should not assume that the user has read all of the documentation. TL;DR.

There seem to be a number of threads in which users have been advised, as with your suggestion, to remember to change "Other Writing Systems" as well as "Latin". Therefore I would suggest that this implementation is not intuitive. What does Thunderbird mean by "Other Writing Systems"? Cuneiform? Hittite Logographic? Linear B? Egyptian Hieroglyphics? If the tag does not have a lang attribute then it seems excessively pedantic to interpret it as anything other than English for the purposes of presentation? Admittedly https://www.w3.org/TR/html401/struct/dirlang.html specifies "The default value of [the lang=] attribute is unknown." and https://www.w3.org/International/questions/qa-html-language-declarations says "Always use a language attribute on the html tag to declare the default language of the text in the page." So I would consider any mail user agent which fails to specify the lang attribute as broken. If remembering to also set "Other Writing Systems" is important, why is "Other Writing Systems" forced all the way to the bottom of the drop down list, rather than being at the top. As I understand the point of turning off HTML, the presentation of the message text of both the received and sent messages would then be entirely in the default font I have set in my preferences, not in the unsupported font specified (probably unintentionally) by my correspondent. My reply would be displayed to my correspondent in the default font of the correspondent's e-mail agent. I sympathize. HTML formatted e-mails are almost always improperly laid out. In particular I believe all mail user agents should complain vigorously if someone uses HTML tables for layout considering that practice has been deprecated for almost a quarter century now and all modern mail agents, like Thunderbird, support CSS3. I would like to understand '''why''' Thunderbird is refusing to comply with my request. Uppity software has always annoys me. I similarly hate traffic signals because we should never have given those stupid blind robots the authority to decide when and where vastly more intelligent and aware human beings can drive and then given them the authority of a human police officer. That is why I prefer traffic circles which leave human beings in control of their own safety and the safety of others. Traffic circles are the Libertarian intersection design.

You make interesting points. However, please remember that those of us on this forum are here to assist users in using TB as it is, not in fixing it or in enhancing it. My intent is to help people who are having difficulties. For example, I emphasize the Latin and Other Writing Systems here because the few people with problems often benefit from setting the major character sets. Most users never need this adjustment, as the default is almost always sufficient. If a user wants total control of the font as they see on their PC, the only solution is plain text, including the Latin and OtherWritingSystems settings, because it is only plain text where an email client can be responsible. HTML is a markup language and an email client can be responsible only for the initial creation of the message. For example, whenever I correspond with people who use Office365, I almost always find that the text I sent is returned in TImes Roman. I sympathize with your concerns: people using HTML sometimes assume that whatever font they pick will be available on recipient's PC and they have no grasp on the often malformed messages they create. I also use Variable Width and have found that my preference is honored when responding to messages sent in other fonts and I regret that, where the font is not available, that you encounter this inconvenience. I tried several ways to attempt reconstructing your issue and I cannot. I do suggest setting a minimum font size and also turning off allowing messages to use other fonts. Those two settings may reduce the pain you feel.

I would like to understand why Thunderbird is refusing to comply with my request. Uppity software has always annoys me.

I see the issue, try setting the value in Composition, not General where display fonts are updated.

Thank you for your interest. I appreciate that you are trying to be helpful as a fellow member of the user community.

"I see the issue, try setting the value in Composition, not General where display fonts are updated."

I do not understand what preferences in Composition have to do with the fact that Thunderbird understandably does not know how to display an unsupported font which is why I am trying to instruct Thunderbird to ignore the font which it has associated with the text and instead display it with the default font "Variable Width" with Size "Medium" which is what is set in Composition preferences.

I am trying to understand by searching through other community discussions but they are not helpful and TL;DR. For example https://support.mozilla.org/en-US/questions/1287677 "it is advisable that you do not send emails with hard coded font and font size. The advised setting to use is this:

   Menu app icon > Options > Options > Composition > 'General' tab
   Font: 'Variable width' and Font Size: 'Medium' "

But that is what I have set. I also have "Send messages as plain text if possible" selected. I have absolutely no interest in using an uninstalled commercial font in my reply just because my correspondent is using a Micro$oft Windoze email agent which insists upon using commercial licensed ClearType fonts. I did not ask for "Calibri Sans-Serif" it is what Thunderbird set and which Thunderbird refuses to stop using even when I ask it to desist. Note that I have "Use Paragraph format by default" set because I like having spaces between paragraphs without having to hit Enter twice. I cannot find any explanation of why Paragraph and Body Text behave differently in Thunderbird aside from separation, even in "answers" to the explicit question "What is the difference between Paragraph and Body Text".

I refer you to the screenshot in my original question. I originally asked this question in June 2021 but received no comments from the community. Thunderbird has set the font for my reply to a commercial font, in this case "Calibri Sans-Serif", which is not installed on Linux because it is licensed. I click on the font in the editor and explicitly request "Variable Width", which is the default set in my Preferences, and without explaining why Thunderbird leaves the font as the uninstalled "Calibri Sans-Serif" and consequently is unable to display my text in a readable format. I object to software, even free software (although I make a donation to Mozilla every year), which rejects instructions without an explanation of why. This is a violation of the Second Law of Robotics:

Second Law

   A robot must obey the orders given it by human beings except where such orders would conflict with the First Law.

How would obeying me and setting the font to "Variable Width" as requested "... injure a human being or, through inaction, allow a human being to come to harm."

Well, to follow the robot dialogue, you have given the instruction to TB to allow other fonts so, in that regard, it is obeying you. And on its attempt to respond with the situation it has encountered, it's a minor play on words to say it is following the Zeroth Law: "A robot may not harm humanity, or, by inaction, allow humanity to come to harm." In that, it is trying to make the situation work.

Okay, that was fun, but doesn't answer your concern. From my own experience, TB always responds with Variable Width set. If interested, you may appreciate the addon, thunderhtmledit, as it provides a tab on compose window to see the full HTML being constructed (and to modify if desired). When replying with what appears as a non-supported font and a too small size, seeing the HTML setting may help understand what the recipient will see. I did send an email to myself with an unsupported font and, yes, TB squawked that it wasn't installed, but when I clicked reply, TB set reply window to Variable Width, as desired, which is not what is happening for you.

You and I follow the same HTML approach: do not make recipient use my font choice or my font size choice. You may have located a bug, but I cannot duplicate it. But I've enjoyed the discussion.

Bear in mind that I am not the one who introduced the use of an unsupported font. The e-mail thread that I am referencing started with me sending a plain-text e-mail to a supplier whose Windoze e-mail client unnecessarily responded with an HTML formatted reply using the ClearType fonts that Micro$oft is so obsessed with. I would rather send plain text messages, but it is Thunderbird which chose to respond using the same font that it received from my correspondent rather than honoring the default I had set of Variable Width. I do not want to override the font, which I could do, because that would mean that my correspondent would receive my message in an explicit font and font size that might be unpleasant. So I want to write back in plain text, as specified in my preferences. The Thunderbird Editor pane has Options > Delivery Format > Plain Text Only, but the editor itself continues to operate in the unsupported Calibri, sans-serif, which Thunderbird cannot display properly because it is uninstalled. Regardless of whether I am sending in plain text or in HTML I want to send messages that do not specify a font so the text will be displayed to the recipient's preferences. It is not necessary to set a specific font in a rich-text HTML-formatted message. You can leave that up to the recipient's user agent. But Thunderbird rejects any request to clear out the specific font, it will only permit me to select another specific font, which may not be supported by my correspondent.

I appreciate that nobody in this community has the authority to respond on behalf of the developers. However this is a long-running issue which has caused other users to lose patience. I have tried to be proactive and polite in my responses, but you seem to be trying to shut me down. How is that productive? It would appear that the development team, many of whom are themselves volunteers I presume, doesn't care whether users can actually use the software. They seem to be living in an alternate universe in which their own perceptions of how Thunderbird works is influenced only by other denizens of their universe. Another user raised a similar issue three years ago and clearly had lost his temper in https://support.mozilla.org/bm/questions/1264871:

dantree 31/10/2019 22:18

This just amazes me, I can not even find preferences unless you are going to do an idiot guide what is the point????

It is really frustrating when nerds think everyone can just follow stuff, wtf fk change what isn't broken? the font is stupidly small, I've increased the size using the up and down arrow then when an email gets sent back to me, because I run my business is stupidly big!!!!!

Really fkd off why can't things just be simple

How am I shutting you down? I'm actively listening and my explanations are not excuses, but only explanations of how it works. If you used plain text mode only, you would have none of these problems. TB can send in plain text using variable width, but when replying to a message in HTML, the response will ALWAYS be in HTML because your setting is to use HTML and the message is in HTML. In that situation, TB normally uses Variable Width and it may be a bug you are experiencing that I cannot duplicate. If I could, I would report it, but reporting false bugs serves no one. You may have found one, but until it can be confirmed, it remains unconfirmed.

If you prefer plain text, that is your real solution. I will offer that, when you have an HTML message and you have the account set to compose in HTML, you can PREVENT it in the compose window for that message by clicking Options>Delivery Format>Plain Text. That resolves the issue. And, it is impossible for TB or other clients to use a nonexistent font. When a font is specified that is not available, TB will use what it thinks is an alternate font. You bypass all of that if you specify plain text as your default.

In these discussions, I have maintained a calm demeanor, while you have been most insulting. You have a problem, I offered approaches to minimize the impact, I attempted to duplicate the problem, I gave suggested changes to your settings, yet you still assault us. I am here ending my participation. The developers who maintain TB do it because they believe in providing a quality product. They are not 'nerds' who want to do whatever they wish, regardless of whether the outcome is useful. Signing off.

Things are simple, but you are first refusing to allow the fact you are quoting HTML which explicitly references the font you are apparently excited about. You are expecting a simple bit of software to anticipate your personal quest to be free. I am sure it will surprise you, but much development effort goes into separating quoted text from the reply so as far as I know conversion to plain text is all you have as an option to remove the font references and converting the thread to HTML. If it is Plain text you like, then hold the shift key when you click reply and reply in plain text. In my opinion that is problem solved as the reply has no HTML for you to not be happy with. To force this go to account setting > composition and addressing and remove the compose messages in HTML format

I think you might also need to look at the font substitutions and the over all quality of fonts in Linux. I have dabbled with Linux since before the start of this century, initially with Fedora and Mandrake. I have even tried using it as a daily computing device. I am back using windows for many reasons but I still hope Linux will get out of it's 20 year GUI beta. Linux fonts are overall of relatively patchy quality and they do not deliver a reliable interface. I simple search of Mozilla's Bugzilla shown fonts to come up again and again in reported Linux only bugs.

The font you are complaining about is likely not your issue, font substitution is (and perhaps size declarations in pixels) have a look at this article https://nolanlawson.com/2020/05/02/customizing-fonts-in-firefox-on-linux/ Firefox and Thunderbird use the same rendering engine, so just substitute Thunderbird for Firefox in the discussion. Perhaps you can set your o/s to substitute something appropriate instead of saying nothing is installed.

Matt, Thank you for your well researched suggestion. Linux, as you imply, remains "not ready for prime time." In particular Linux developers must remember that on laptops and desktops their systems will inevitably be existing in a Microsoft/Apple ecosystem and it is pointless to pretend otherwise. The link you reference is helpful but as in many aspects of Linux there is insufficient documentation for mere users to address this issue. Since the introduction of Windows Vista in 2017 Microsoft has entirely migrated from its 1990s commercial font family, which included Helvetica etc, to the ClearFace family. It would be nice if this had been recognized by the fontconfig master fonts.conf file, but that is one of the pointless time-wasters that Linux is prone to. Linux is "free as in speech, not free as in beer."

However that was never my question. My concern was 'Thunderbird ... ignores requests to change the font to "Variable Width"' Adding support manually to the fontconfig would eliminate the warning message about the lack of an installed Calibri font, and provide me, personally, with more control over the appearance of messages received from Windoze email agents, but would not address the violation of the 2nd Law of Robotics. I gave an order to the Robot Thunderbird to change the font of a paragraph from Calibri to Variable Width and Thunderbird ignored that order without explaining why it was ignoring the order. The fact that David has been unable to reproduce this behavior indicates that there is something contextual about this behavior. But if Thunderbird will not tell me WHY it is refusing to obey my command I cannot productively raise an issue with the developers, and I cannot change my behavior to achieve my objective.

In addition to my first report almost a year ago now I have also pointed out a similar complaint from three years ago which the user found so upsetting that he, unlike myself, felt driven to the use of unproductive profanity.

Linux is a collaborative environment, and if I can figure out how to alter the fontconfig master file to support an environment dominated by Windows Vista, 7, 8, 10, and 11 I will submit that proposed change to the community.