Prin, the mail folders you lost were local folders - since you were/are using POP - so, I'm wondering if somehow Thunderbird became disconnected from your profile, so it appears that the mail is lost, when in fact it may still be there. (The SMTP issue is likely not related).
As per Jim's first link, have a look to see if you can find your TB profile files as a first step. As is noted in that link - the app and the profile are separate, and an update to the application shouldn't affect the profile or files within it.
More - example is for Windows, but I think the principles are the same on a Mac. Apparently this is NOT an unknown issue with TB.
https://www.emailadepts.com/blog/how-to-recover-lost-folder-in-mozilla-thunderbird.htmOut of curiosity - any reason you're not using Mail? I switched to it over 10 years ago after being a long-time Entourage user, and I must say, despite a few complaints, it's actually worked pretty well for me. And I currently have 32 email addresses...and thousands upon thousands of emails in there. (many of the email addresses are connected to clients' domains - I'm not completely insane...yet)
BTW - if you do find your profile and mail folders, PLEASE back them up - copy them to a hard drive or a thumb drive or iCloud or SOMEWHERE (maybe twice) to be sure that you have them safely and then, and only then, start attempting the fixes to re-link the app and your profile. And get that backup drive up and running - whether as a clone or with TimeMachine.
(I usually tell people to have both. Drives are cheap - your time and your data are not).