Why are you running 32 bit anything on a 64 bit machine? Nothing has been altered at all - upgrades, power outages, etc? earthlink+AOL+verizon could be a raft of issues - could you provide any more details?

Thanks for the reply, jeff-g. To your second question: no, nothing changed within my awareness. Just stopped working. Third question: I've used three e-mail providers for many years, for a variety of reasons including security (anonymity + throwaway addresses), control (I own two of the domains) and separating work from personal mail). Never had any interferenc, Thunderbrid always seems happy with sending to multiple SMTP servers and receiving from two dozen POP accounts.

First question: I'm no IT expert, and have often wondered about this. But the download pages I've used never showed an obvious choice more fine-grained Windows (though now it has a separate entry for earlier Win versions).

Anyway, I'm not sure how I got on the ESR channel - not by my choice! After making a few backups, I downloaded the latest regular release-channel version, which turns out to be 140.0.1 (64-bit). That's a lot of updates I'd apparently missed!

Good news! (to quote Professor Hubert J. Farnsworth, but without irony): it seems to be working fine on all POP channels; haven't tested all the SMTP servers yet. I don't know if it was simply too old a version for some functional compatibility, or your 64-bit observation was the culprit, or something else entirely. But OK for now, and very much relieved! Even made another donation to the volunteers here.