try toggling hardware acceleration in the preferences. Try restarting from the help menu (troubleshooting mode..) and continue when prompted. seeing the same issue in safe mode?

Thank you Matt for answering.

I'm already relying on the safe mode, where fonts are normal again.

When in default mode, I don't have any extensions running... or any themes active other than the one that thunderbird came with.

Don't see any hardware acceleration toggle in the prefs. macOS specific absence ??

With Firefox, it has been the same problem when it had a big version update hitting the 90.x mark.

It's a nightmare to have to restart into safe mode just to be able to get readable fonts instead of having everything going extra-bold.

Am I the only one with this issue ? Have been on Thunderbird since the Netscape suite went obsolete. Decades ago ...

Is there a way to start a fresh instance of TB without losing my profile and all the content ? Like using App Cleaner to get rid of rotten dependencies ... but preserving my account details ?

Any help is appreciated. Thank you again Matt.

Hardware acceleration is set in Preferences/General/Indexing. Disable it in normal mode and restart TB in normal mode.

Acceleration is disabled in safe/troubleshoot mode.

Found the HW accel, last item in the General tab. Thx.

In normal mode, HW acceleration was/is not enabled. Just for test purposes, did toggle it, no changes.

Mystery still open-ended.....

Launch in safe mode, 'Reset toolbars & controls', 'Make changes & restart'. Better?

Did that yesterday, didn't cure from the Bold Font Syndrome.

Also checked the No-Add-Ons option...

But there is still something changing this UI behavior when switching to the troubleshooting mode. Even with no Add-Ons on board. What could be happening under the hood ?

Still, thank you for providing your assistance, much appreciated.

The other possibility is you have a userChrome.css that bolds the fonts (css is disabled in safe mode).

If that doesn't help, try rebuilding the OS X font cache.

Interesting....

Can the userChrome.css be "repaired", edited or manipulated in some way ?

I'm no programmer, my knowledge is rather basic. Is this css file present in the config menu ?

You probably don't have a css file, as it has to be manually created and placed in a subfolder of the profile named chrome.

Unless you're using a non-default theme in normal mode, I don't know why safe mode eliminates the problem.

You could test with a new profile starting from Help/More Troubleshooting, about:profiles, and add an account. The current profile is unaffected, and data can be transferred from the old to the new if the new one works properly.

Finally !

Thunderbird pushed a new version, 91.3.0 and with this one, my Fatso Font problems went away.

Something was broken, still can't put a finger on it...not geek enough. I tried everything presented here, even set up another fresh Mac account. Fonts stayed bold as hell.

btw ... the font cache cleansing & rebuild was without effect, repairing permissions doesn't exist anymore among the commands of the terminal, syntax not recognised. But hey, that article was from 2015.

Now, if the Mozilla folks could do the same pro active repairs on Firefox, that would also eliminate the same Fatso Font Syndrome on the browser, broken when it too got a major update.

Thank you guys for your help and support. It was worth the wait.