Do you see any errors mentioned by thunderbird?

No sadly not

You might try to remove the password saved for that account (options/privacy & security/passwords - show passwords and here to delete the pass saved for that account) restart thunderbird -> when it will reopen it will ask you for the password for that account - enter it + ok Will that show any error?

Thanks will try that

I think you can see if there is any "error" or if just thunderbird did not found any "new" messages on tools - Activity Manager... What it show there - no messages found or an error, something?

No luck. I’ve uninstalled thunderbird and removed all my email accounts. Reinstalled and went to add an email account and it comes up with “Thunderbird failed to find the settings for your email account.” Which is never did before. I then go to “Advanced Config” option and allow it to create the account anyway. I can then send, although it won’t save the email in sent, but I cannot receive. Very strange!

I upgraded to v. 78 last night and had the same problems, no incoming e-mail, and got the same error message when trying to fix it. Since I have 4 Macs, I decided to revert to v. 68 which was still working just fine. Got no replies to my how-to-revert query on this support page, but finally figured it out. Reported here in case it will help you or others, since apparently this new Thunderbird version was obviously not ready for prime time.

I just reverted to v.68 by copying a working version from my laptop MB Air to my desktop Mini (both running Mac OS 10.14.6). For those interested in reverting until all the bugs in 78 are fixed, just a couple of steps are involved. You must be copying from a working v.68 on another Mac. First, delete the application; I also deleted the dock icon just in case. Next on your destination computer, in user/library delete the Thunderbird folder (mine had 6 files in it). Your address book is in there so you won't get it back, you'll be substituting the version from v.68.. Copy those two items (the Thunderbird v.68 application and Thunderbird folder from the user/library) from the other computer into the same respective locations in your destination computer. Thunderbird should launch and work properly (mine did after doing this, thank goodness).

Unfortunately I don’t have an older version. Any other way of rolling back the update?

You can install the old version - You go on tb download page and get 68.9.0 https://archive.mozilla.org/pub/thunderbird/releases/68.9.0/ Download software and install it. On last step of the installation do NOT select to start tb After installed go to run (search run, select that run app) and write on that small field this: thunderbird --allow-downgrade Press enter and that's all. You will have 68 working. After that go to Option - Update - change update to manually and disable update service. After that you can search for updates manually (should suggest 68.12 - this one you can update). Please note that you will need to search on addons page the versions which are working with 68.0* - those with 78* will not work

Do you run regular backups? (I should have thought of mentioning that above). You can certainly restore V.68 from a backup. It will be missing any changes since the last backup, but at least you'll have a good working version, and if you use IMAP, all the messages will be there.