did you actually install Thunderbird? Most apple users with your complaint have simply run the DMG file and everything is saved in the system temp file only.

I've run the DMG file as well. How do you actually download Thunderbird ?

Hi Matt! Hi Ad2souza - Matt this is a problem with Big Sur (aka Big Nightmare) and the "keychain." I have MS Outlook on my MacBookPro running Big Sur. My Outlook CONSTANTLY asks me to type in my password for each of my email accounts using this client. And with the gmail I have to go back and give the permissions, over and over and over. I'm so sad to read here that Mac Users running Big Sur are having this same problem with Mozilla's Thunderbird. The future for those of us using a 3rd party IMAP email account (eg Earthlink or you company server) does not look bright in MS Outlook. So IF Thunderbird is still supported by Mozilla, I'll migrate on over. My thanks to Ad2souza. Intendo espanol, pero non puedo escriber muy bien. In any case I'll post on the Apple site that we have this problem. Let's just hope that those over-paid engineers at Apple (the software engineers are still in CA aren't they?!) can publish an update to this keychain issue.