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Steps to reproduce:
I have an email account for which I do not save my password in Thunderbird. Instead, each time Thunderbird starts, I type this password. Until fairly recently, typing the password once each time Thunderbird started was all that was required, as Thunderbird apparently hung onto my password until it was closed and restarted (at which point I had to re-enter my password again).
After a recent update (and I've tried a few more updates since then), Thunderbird seems to forget my password after 10-15 minutes. Or, if I click on a sub-folder, I'm often asked to re-enter the password. Or, if I step away from my computer and upon returning click on "Get Messages," or a different folder, I'm prompted for my password again. BTW, in the past I would have never had to click on "Get Messages," as Thunderbird would be checking my IMAP server throughout the day, but after this timeout I notice that mail I was expecting hasn't shown up, and I think it is because of this password issue.
I think when the new encryption methods were put in place, Thunderbird lost the ability to keep a password in memory for any period of time. I do not know if this is by-design.
I'd hope Thunderbird could keep a password I key-in until such a time as Thunderbird is closed. But again, this may be by-design and a "feature" that is no longer possible.