If this only occurs with messages from some correspondents, it may be due to the email formatting done by the other person. Some email clients promote their ability to change font and background color (and this can be done with Thunderbird with radical customization). My suggestion, before investigating any possible bug in Thunderbird, is to browse the html as it arrives. Also, if you are using an addon theme, try reverting to one of the themes that were provided in the release and disable any addons that change fonts or colors. If nothing seems to work, I'll be glad to review if you redirect (not forward) to me at davidthunderbird at gmx dot com.

Thanks for your reply! I have tried a variety of themes, all provided with Tbird; I don't think I downloaded any special ones. They do not affect the fonts and colors of the body of incoming messages. But some messages, like David's from support.mozilla.org do not display the black background etc. And I see a major difference between the two types, though I have not examined all of them.

I saved samples of the two kinds and viewed them. David's displays a copy of the body as plain text, and below that as HTML. No problem.

--===============5843143595972678183== Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit

Hi ronks1, ... and so forth

--===============5843143595972678183== Content-Type: text/html; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 8bit

... and so forth The ones that give me trouble with tiny fonts, some in nearly black colors, are like this: Message-ID: <1406077828.1522294.1704216604147@aol.com> X-YMail-OSG: 87_D1QUVM1nd4Awmj2tUyWMptN4QuxGVx51tJR1VNwo0.HaZbGDYR4miuO2z0Ny ... and so forth ----_com.samsung.android.email_234468053267310 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: base64 VGhhbmsgeW91IFJvbi7CoCBTbyBzb3JyeSBmb3IgaGVyLsKgIEkgd2lsbCBkcm9wIGhlciBhIG5v ... and so forth I suspect this difference is the key; but how to address it is unclear.

Another data point. If I save a problematic message as an .eml file, and open that file with Tbird, I see plain black text on a white background, no formatting. Frankly I would be happy with that; but it is a cumbersome 2-step solution to use for every such message.

And yet another; in fact, two. Running TBird in Troubleshoot Mode does not cure the problem; same body colors and font. But it does not occur at all on my Windows 11 laptop, which is basically configured the same. I wonder if I can re-install TB on the desktop Windows 10 system without losing my settings and the like.

I still suggest going slowly, as if the bad formatting happens only for a few messages for particular individuals, the problem may not be with your installation of Thunderbird. Installing again rarely fixes issues, and any change can always go wrong. You could install a fresh copy over the top of what's there, but I encourage doing a backup of profile first. The fact that some messages appear properly indicates that may accomplish nothing.

I think I have got it. I compared the (good) laptop settings with the (illegible) desktop ones. Under

  General  |  Fonts & Colors  |  Colors

I checked "Use system colors" which was unchecked on the desktop, and that seems to have restored visibility. Would still like to have white text on black background, but not going to mess any more for the moment. If it ain't broke, ... Thanks for your help!

I'm glad it worked out well. :)