Thunderbird only works in safe mode

[triggerfish]

I whacked W10 and have a new clean install of 20.04 on a Lenovo Flex 14 and pointed Thunderbird at my copied (old) profile directory, with 5 or 6 email accounts (yahoo, gmail, my business etc) and all associated data. Thunderbird will not either retain passwords or connect to any of these accounts. However, if I start it in safe mode it magically works OK. My workaround is to operate it like this until I find n aya solution, but it's not ideal. I've hunted around and tried various suggestions but none of them work. Found safe-mode solution a bit by accident.
Can anyone suggest a solution?

[WaltS48]

What version of Thunderbird?

Extensions that worked in 68 will no longer work in version 78. There may be replacements.

Have you tried opening the Add-ons Manager and checking for update to the extensions, disabling half of the extensions and tried to get mail? If that doesn't work try disabling half of the remaining extensions until it works, then track down the one that is causing the issue.

[triggerfish]

It's version 68.10.0. I only have one add-on (FileLink for WeTransfer) and that is disabled. But even with this disabled I still have to run in safe-mode. I can't see an option to remove the add-on, unless there is a config file somewhere. The process to get it to look at my old profile was a bit tortuous. I created a new profile and then substituted the new profile directory for my old one. This probably has nothing to do with the problem, but of course there might be an ini file or something that is out of line.
In the Safe Mode dialogue box if I select 'Disable all add-ons' I can't get the 'make changes and Restart' button to work (I have to 'continue in Safe Mode), so perhaps (this being Linux and the fact I have monkeyed around with the profile) it's a file permissions issue??

[WaltS48]

To remove WeTransfer go to Preferences > Attachments > Outgoing, select it and click the "Remove" button.

Restart and see what happens.

Starting in safe mode also disables hardware acceleration if it is enabled. You find that under Advanced Configuration in Preferences > Advanced > General.

Mine is disabled.

[triggerfish]

The Outgoing panel you mention doesn't show anything and my hware acceleration is off. But in the Help->Troubleshooting information it lists 8 !! extensions (Amazon, Bing, Chambers, DuckDuckGo, Google, Twitter, Wikipedia, and our old friend, the one I can actually see in the add-on manager, FileLink).
I installed Tbird via the Kubuntu 'Discover' software centre. I might try deleting and re-installing it. It seems to be a half-baked, unless I'm missing something (which I probably am).

[WaltS48]

I also have the FileLink extension in the Add-ons Manager.

If I disable it, the WeTransfer I see in my Attachments > Outgoing disappears. Clicking the Remove button doesn't remove the extension.