It's probably the same issue as explained here: https://techdows.com/2019/06/avg-reportedly-deleting-passwords-and-logins-saved-in-firefox-67-0-2-fix.html

Fixing it will be very similar.

While I understand that there must be a corrupted file somewhere, what file should I look for and where?

Select Help/Troubleshooting Information, click Open Folder to open the profile folder in File Explorer, close TB, then make the changes to the logins.json files as described in the above link.

You may have to allow the viewing of hidden files and folders in FE.