EDITED POST
After doing a bit of googling, the problem wasn’t with TB or FF, as I first thought, but with Ccleaner, or should I say, with its latest update.
5.31.6105
I thought I would do a heads up on the latest update for ccleaner,
the best thing is to uncheck all the boxes in Ccleaner to clean FF and TB settings, until this bug has been fixed. but I’m not going to take my chances and unchecked the lot. I would advise anyone who uses ccleaner to do the same before they update to the latest version.
I have been using ccleaner for many years without issues, but after the latest update, it wiped all account data for FF (tabs, extensions etc) and thunderbird (email accounts) , so all my shown email accounts, emails etc was no where to be found.
The good new is that I managed to recover some of my accounts, by going to
C:\Users\username\AppData\Local\Thunderbird\Profiles(profilename)
and look for prefs-1.js rename to prefs.js (don’t bother trying to use the invalid prefs file, as that won’t work, if you don’t have a backup, then you need to recreate all you email accounts from scratch.
it seemed that my old prefs file, was renamed as invalid, and could no longer be opened. But lucky I was able to rename another profile js folder, which partially restored most of my accounts, then it was the matter of re adding the accounts that were left.
if you use TB, BACKUP your profile folder, and make sure you have a working JS file, so you can easy rename it, and place it back.