Well, I've been playing around but adding the data to the wrong thread, but I'm sure they are related:

https://support.mozilla.org/en-US/questions/1074170#answer-759631

I switched back to the older version 31.8.0.

And I just cleared my computer caches & restarted the whole computer, and its still happening, in Safe Mode, in this earlier version 31.8.0

And continuing in Safe Mode -- when Replying, a test I just made started out with my typed font being the same size as it ought to be; the same size as the quoted material.

I have Min Font Size set to 20pt. So everything in my Reply is displaying larger than in the received email. So far this is correct behavior.

BUT when I started typing another line after hitting RETURN, the font size became smaller.

And even when I moved my pointer back to the initial line and typed more, it remained smaller.

So before I waste more time trying an even earlier version -- is there something in the SYSTEM Library files (MACINTOSH OSX) that does not get scrubbed when changing versions or installing another copy? What do I need to manually go in and purge?

I purged the USER/Library/Caches file, which is all I ever need to purge to fix slowness, according to my Mac expert.

This is still happening, even in Safe Mode. So can't blame the Theme & Font Size Manager addon.

I think its doing one font size smaller than any quoted text.

And even though I have set Prefs for sizes of 20 pt then down to 12 point, recipients of email tell me they are seeing 32 point. Which is much larger than anything I have ever set for a Pref or a Minimum Font Size.

Also, the problem of main menu pop-up boxes being too small to see all of the contents, and of their getting smaller each time I click on a different sub-menu, continues to occur in Safe Mode. See image attached.

So something is very fundamentally effed up since the last Update.

Now I think I need to completely un-install Thunderbird and start over again. But how can I save my Mail and AddressBook, and drop them into the newly installed copy, so I don't lose it all?

I have in front of me an email exchange with 6 levels of quotation.

EVERY LEVEL is a font smaller than the one above.

Something is really screwed up with this version of TB !!!!

I need really clear instructions on how to wipe it out & reinstall from a new disk image, while saving the essential data, and how to drop that data into the new install.

STILL HAPPENING ver 38.4.0 MAKE IT STOP !!!!!