I don't have time today to fully respond, but I'll give you the basics that WORK. - leave the variable width HTML setting UNTOUCHED. All of what you want can be done from plain text. - in tools>general click colors and set colors there. do NOT tick systems colors. this sets the colors YOU see. dabbling with the HTML settings sets colors OTHERS see. - in tools>general, click advanced for fonts and use the dropdown menu at top and set Latin and then Other Writing Systems to be IDENTICAL. Completely IDENTICAL.

I attached samples of my setting. With these, you see the same font in compose and sent folders, plaintext and html messages. There is NO way to my knowledge to set color for plain messages separate from html messages unless you use the HTML color settings that cause sent message to use that color (which your audience may not appreciate). My settings also are such that recipients never see the font and color you like, which is how I like things... :)

Thanks, that does seem to help with the quote color. I didn't realize that setting the color for quoted text in received emails would also set it for my own quotes when sending. That is very unclear from the way the settings are labeled.

I still have one problem though. It looks like you achieved "compose plaintext in proportional width" by just setting your monospace font to a proportional font. I think that will mean that even monospaced text in received HTML emails will display in that proportional font, and I don't want that. I don't want Thunderbird to "think" I am composing plaintext emails in a monospaced font, and then fool it by actually setting that font to a proportional one. I want Thunderbird to actually not attempt to use a monospaced font for composing plaintext emails in the first place. Is that possible?

A monospaced font is one that has no styling, the characters as they appear - plain text. It is impossible to put styling on plain text or for plain text to have a native font of its own, so making it appear proportional requires the email client (whether Thunderbird or others) to control the font. You're not 'fooling' the email client; you're specifying how the characters should appear. Thunderbird anticipates encountering plain text and is asking you how you want it presented. I fail to see the problem.

The problem is that what I want is to control how plain text is displayed, not how monospaced text is displayed. As you say, plain text has no native font. When I receive a plaintext email, that text has no font information. Then Thunderbird somehow has internally decided that plaintext email should be displayed in a monospaced font. Then it allows me to choose that monospaced font. What I want is to override that earlier decision, and tell Thunderbird, "Plaintext should not be displayed in a monospaced font at all in the first place. Don't tell me to choose what monospaced font to use for plaintext. Let me tell you what font category (serif, sans serif, monospaced) should be used for plain text."

Another way to put it is that choosing my preferred monospace font in Thunderbird, right now, is specifying two things: it specifies how I see plain text emails, and it also specifies how I see explicitly monospaced text in HTML emails (e.g., if an email uses a <pre> tag). I want to separate those, so that I can still see things like <pre> tags in a monospace font, but see (and compose) plaintext emails in a proportional font.

Thanks again for your help!

Thanks for clarifying. Now I understand. The issue is that both situations are monospaced font, whether stand-alone or within HTML. Thunderbird isn't deciding anything; it's showing you your options. That panel could have said 'Plain Text' instead of 'Monospaced text' and the actions would be the same. Anyway, you now know the options, so I wish you well.

Yes, I understand the actions would be the same with a different label, but that's the problem. The problem is that "plain text" and "monospaced font" are two different things, but Thunderbird conflates them. It is quite possible to display plain text in a proportional-width font, but Thunderbird doesn't want to do that. The only way to do it is to fake it by telling it that a proportional font is actually monospaced. So it seems the answer to my question is that there is no way.

Thanks again for your help.

You're wanting two types of plain text: one that can be shown in a predefined font within HTML messages and one that can be presented in a predefined font for messages that are not HTML. I am not aware of any email client that does that. If you want to see plain text under the HTML preformat tag, then your best bet is to set HTML as your default and set monospace font to a monospace font.