Sounds like you are another one using Theme and Font changer add on. It seems to have developed a problem lately.

There have been several postings like this today. Are you a Mac user? Had you been using the Theme & Font Size Changer add-on?

Those of us who use Linux have had to find other ways of doing this since support for the Linux version of that add-on was dropped in the last couple of years. I recommend you set up the userChrome.css file for your menus etc.

Another file, userContent.css can do the same for message content, though you do have specific adjustments for this under Tools|Options|Display|Formatting→Advanced

Here is how to do it using userChrome.css:

https://support.mozilla.org/en-US/questions/1198055#answer-1059537

The specific case being addressed there was for the new (Quantum) version of Firefox, but it works in Thunderbird too. So you'd do all this in your Thunderbird profile.

Many people have reported this add-on failing today. You should contact the Add-on "Support Email" for https://addons.mozilla.org/en-us/thunderbird/addon/theme-font-size-changer/ - namely [email protected]

Note, most addons have a support links/contact listed on their addon page.