I found the solution myself. It is the file prefs.js containing the info. I changed the value for layout.css.devPixelsPerPx back -1.0 and Thunderbird worked again as before. Great sigh of relief.....

Newletters2, thank you, thank you, thank you. My wife wanted a larger inbox message list font on her new laptop. I experimented on my laptop first. I found via google to change that pixelsperfx setting and changed it from 1.3 to 2.0. Huge letters like you said. Could not move them. Reinstalled Tbird over the top. No help. Uninstalled and reinstalled. Found your reply and changed the setting in prefs.js like you did. Success. Everything is back to normal. How on earth could they allow us to make a change without a simple way to restore the setting when we cannot get into thunderbird to make the change? Thank you very, very much for posting your solution.

In my reply I neglected to say the uninstall/reinstall resulted in the same unusable tbird. Navigating to the profile and changing prefjs was the fix, as you stated.