PS: Once I posted my comment, I saw that the topic has been raised on this forum in the past, e.g. here (2018) and here (2015). In the latter, “Zenos” summarised my thoughts perfectly:

“But I do wonder why I'm forced to use the straight quotes. To me they're unprofessional and tacky-looking. I'm not using an old Underwood with a cloth ribbon. That's what those marks remind me of. It's the new, flexible 21st century, Mozilla!”

But in the latter,“ jscher2000” - Support Volunteer (Top 25 Contributor) helpfully provided the following, referring to macOS, although the shortcuts are slightly counterintuitive:

To manually select smart quotes, type the following keys simultaneously:

OPT + ] = ‘ (left) SHFT + OPT + ] = ’ (right and/or apostrophe)

OPT + [ = “ (left) SHFT + OPT + [ = ” (right)

He took that from a (Lifewire https://www.lifewire.com/typing-quotes-apostrophes-and-primes-1074104) article which was updated in November 2022, and which also provides instructions for Windows.

Thank you!

Might I refer you to the 22 years old bug requesting same. https://bugzilla.mozilla.org/show_bug.cgi?id=145765

Then there is a request to make quotations marks language dependent. https://bugzilla.mozilla.org/show_bug.cgi?id=234800

The first bug is in the Mozilla platform (shared Thunderbird and Firefox) the second in for Thunderbird core.

Perhaps let the team know how you feel about this bug languishing for more than 2 decades by posting an idea or offering some feedback here. https://connect.mozilla.org

I'd be happy to, but given that 20 years have gone by without Mozilla bothering to fix the issue, I'm not sure that explaining how I feel about it would make any difference.

On a similar or related note, others have also complained about another Thunderbird quirk, being that some emails are displayed as awful plain text. It has been discussed at https://support.mozilla.org/en-US/questions/1341300 and https://support.mozilla.org/en-US/questions/1422543.

Regardless of whatever truths are mentioned by the moderator(s), the inescapable fact is that no other email client renders those particular emails in plain-text; they appear as HTML. In any case, none of the comments in those two discussions seems to propose any solutions, and hence this is yet another disincentive for using Thunderbird.

I also cannot understand why, having set my default display font to Times (for serif) and Helevetica Neue (for sans-serif), emails rendered in HTML consistently display in Segui font (including the one you just sent), and despite having unchecked the box "Allow messages to use other fonts“ in Fonts & Colours > Advanced.

What am I missing?

I must again write a follow-up note to what I just complained about (the plain-text issue). Comparing messages from the same sender with the one rendered in plain text, I now see that the plain-text emails were sent from an iOS device. Those that he sends from his computer at word are rendered in the appropriate HTML format.

Therefore, while the plain-text rendering is the “fault“ of the sender (and not Thunderbird), it's also true that Outlook and Apple Mail just do a far better job of displaying messages sent by iOS devices (and possibly Android devices) than Thunderbird (which does a surprisingly poor job at displaying such messages, being utterly unable to render any part of those messages in HTML.)

I THINK what you are actually asking for is for the ability to type straight quotes and have Thunderbird change them to curly ones. There's nothing to stop you from typing curly ones manually.

Whilst I share your view to a degree, programs changing what I've typed is OK providing they get it right but they don't always and you end up with things like “They don’t like it up ‘em!” rather than “They don’t like it up ’em!”

Using straight quotes doesn't bother me very much as I go back to the days when they were all that most typewriters and early computer printers had.