You can set all the options to use Tahoma, or whatever. Thunderbird won't support your choice of size. 12pt is one explicit size but surely you must agree that it only makes sense if seen on a device very much like your own, or printed on the same size of paper that you composed it for. (If someone prints a message designed for A4 and scales it to A5 or A3 then the font size will scale accordingly.) What should a system do with a specification of point size if it has a small screen (phone) or a large screen (projector)? You'd quite properly expect the text to be scaled up or down to suit. So your 12pt is unhelpful and unfeasible and very often will be ignored entirely. Leave it set to "medium". and let your correspondents choose whatever size suits them.

In plain text messages, there is no font specification within the message. If a particular character set is being used, then you may need to choose a font to display it that provides the appropriate glyphs. So a font designed specifically for latin languages can't display, for instance, hebrew or arabic or any oriental language. So "fonts for" allows you to choose fonts for messages using "foreign" character sets. I'd ignore these settings until you get an ugly message, then you can inspect it to see what character set it used, and then choose a suitable font to work with it.

Most of this is now redundant with the widespread uptake and use of Unicode and UTF-8. Unicode compatible fonts have a very broad range of glyphs to cover a large number of human languages. Choosing fonts for specific character sets and encodings refers back to those clumsy days of "code pages", when you'd have to configure or optimize your entire PC to suit your working language.

HTML mails don't have this problem since they can embed font specifications, though Thunderbird allows you to override these. Of course, your computer, in general, can only display fonts that it has installed on it, so exotic or specialist fonts are somewhat futile in email.

Having said that, Thunderbird continues to have its own peculiar problems with font sizes in HTML messages. I find that the Stationery add-on helps, by allowing us to specify paragraph formatting. I also like the Extra Format Buttons add-on, which allows you to select text and apply a font size. This, for me, is principally to deal with correspondents using Outlook, which ignores Thunderbird's formatting and asserts its own, making your contributions to a conversation look rather over large and silly. This button lets me repair the damage.

More on fonts in Thunderbird here: http://xenos-email-notes.simplesite.com/417754775

Your elegant response is greatly appreciated, though I'd hoped there was a simpler, existing list of best-bet settings somewhere in Mozilla's knowledgebase. Obviously not, but you've sent a whole education which I'll have to pursue. Thank you!

A common piece of advice to help unwind troubles with fonts is to "set them to the defaults", but that's easier said that done. It really just re-asks your own question, since the default settings aren't documented anywhere easy to find.

I meant to say, but forgot, that as for incoming messages, the only sensible thing to do is let Thunderbird choose the encoding declared within the message; forcing a different encoding leads to mis-displayed characters. But you can usually override the message's font with another of your own choice. An older font (possibly Tahoma) may not be unicode compatible. Tools|Options|Display|Formatting→Advanced|Allow messages to use other fonts, if cleared, will set Thunderbird to use your choice of font in preference to those stated in the message.

Some users get persistent "funny characters" in incoming messages and it's not clear why this happens. User have also reported these characters appearing after a message is saved as a draft and then re-opened. It usually suggests that a message was encoded in UTF-8 but displayed with a different encoding. The mystery is why this inconsistency happens.