153.6.0esr crashing on start

I'm running 153.6.0esr on Windows 11. I recently signed up for Fastmail (using IMAP) and since adding that account, Thunderbird crashes ~90% of the times I open it - just perma hangs before it can ever show all my mailboxes. This is curious because I have a different IMAP account (different email provide) that has never caused this behavior. Starting in Troubleshoot Mode doesn't help. I tried checking off "Check for new messages at startup" on that account, but whenever I try to load Thunderbird, I can see it stuck (in UI text at the bottom) checking Fastmail servers... Occasionally I get lucky and Thunderbird does load ok, I can then use it ok. But even in that case when it can load, *sometimes* it loads my Address Book, but sometimes it thinks my Address Book is empty. Fastmail would only advise me that I might have a corrupt Thunderbird profile. Fine, I guess I'll try making a new profile, but I have a few questions: 1.) What might have caused this corruption? Clearly it was something about adding a new Fastmail account (whether Fastmail specific or not). Is it possibly something to do with Address Book? Is there some other fix I should try? 2.) Even if a new profile works, how do people usually deal with importing other old accounts? I have a number of big old POP accounts and I need to have all that email around to reference. Do I just try copy and pasting those mailboxes into the new profile? Thanks.