In article ,
says...

I guess I didn't make it clear that I was talking about the standalone
Thunderbird, not Firefox.

And while I haven't tried it yet, I ran across the below in another
newsgroup. Sure sounds like my problem.

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5. I checked again, and restarted Thunderbird. Then I had a closer look
at the directory structure, and under Profiles\ a directory with varying
name (like 60fth4de.default\) contains the 'real' data. I copied Mail\
and ImapMail\ from my old directory there, as well as the collections of
files found there in the old directory, and voila! everything was
working
again! Even my addresses reappeared!
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BNSF = Build Now, Seep Forever