It may be that something is preventing the prefs.js file from updating. Anti-virus?

I haven't changed my antivirus settings in years, and this problem is new within the last six months. Where do I find the prefs.js file?

- click help>troubleshootinginformation - scroll down to 'profile folder' and click 'open profile' - exit thunderbird - you are now in the profile and the prefs.js is there. It is a simple text file, but I encourage copying it before any attempt to edit it. If you see multiple prefs files, that is an indication that it is having trouble being updated.

I copied the top prefs.js then deleted all 43 copies of it (other than the copy of the original). Now I just have to set up all my email accounts from scratch. Not the best solution, but it works.

If you are using an antivirus product, I encourage you to see if there is a setting to ignore email software. Some have that feature.