Do you have more than one AOL account in TB?

No, do not...only 1. I do have my files saved to my hard drive so I can access when I'm not online... I figured that would take up some but not a 100GB.

At first, I thought there might be a 20, 000 email limit per file/folder and that's why it was rolling over into the – 1 and – 2 folders... Maybe the new ones were created when the others maxed out.

But I'm looking at the file structure now, with the -1 folder removed and I suspect all these new emails that are being downloaded or going into the -2... As I don't see a new folder created.

If TB downloads the rest of these emails and the GB stays around the same, then I have to assume the -1 had some corruption as you may have previously pointed out.

Is your issue now fully resolved?

Is it possible that during a compact operation you either ran out of disk space or interrupted a compact by killing Thunderbird or restarting your computer?

The only bug reports with activity in recent years are these four https://mzl.la/3p2z1Dr

Plus this Bug 553165 - Provide automatic function to delete nstmp files (maybe on startup and/or shutdown) https://bugzilla.mozilla.org/show_bug.cgi?id=553165