Never mind, as usually happens within half an hour of asking for help, I found the answer. My ISP has had an expired site certificate for a couple of years now, I used to be able to store it permanently. Thunderbird decided at some point that permanently actually means every other day or so but would create an annoying popup to tell you that. A few releases ago, the annoying popup got replace by a silly little window that only stays active for a few seconds and if you don't see it and click on it, you had to restart Thunderbird until you were quick enough to hit it. I guess this "feature" is gone completely and I now have to add it manually in the Manage Certificates section that took me a quarter hour to even find. Thanks, Mozilla for making life increasingly stupid and wasting hours and hours of users' time and sanity.

just change the connection encryption to none. The certificate is only offered or asked for with SSL (obsolete), TLS and STARTTLS encrypted connections.

Then move to a mail provider that will actually manage their mail system instead of ignoring it.

Just as an aside, downgrading is asking for long term issues and should not be embarked on lightly. I put up with Thunderbird not working at all for about a week once when an update went bad rather than downgrade I am that leary of it.

Lord only knows what data is lost in the process of a downgrade as the developers do not code to support profile downgrading so if a storage file format has been changed in an update, downgrading tends to loose the stored data as it is assumed corrupt by the old version. So downgrading rarely is a truly good idea.

That didn't fix it. Before trying it, I re-exported my profile folder because I'm waiting on my ISP to call back and I have little doubt they are going to screw something up (did last time, took a week of mind numbing googling to fix what they did). As it started to export, incoming email started working, I mean like WHILE it was copying to a jump drive. I did nothing but open the Profile folder, right click on it, copy and paste to the jump drive. It download every bit of what I can see is sitting on the server through webmail. I haven't been able to send email since Apr 10 from my desktop. I'm pretty sure it's that certificate and I'm also sure that they aren't going to fix it. I can't figure out why saving the exception isn't working, it has worked for years (in a half-assed way). Thanks for the suggestion though, it would've been days before I thought to try changing the encryption.