Ok, resolved the issue changing the configuration item "security.tls.version.min" from 3 to 1. (just go to options / general / config editor / search for security.tls.version.min)

Problem solved.

Thanks

Thunderbird is well and truly screwed for me! It point blank refuses to read my main imap server and all the TB folders have disappeared. I have used webmail to check the folders on the imap server - all appear normal. Two other connections to Gmail accounts are working ok.

In desperation I installed the hated Outlook using the ISP's recommended settings. Outlook behaves as expected.

So, what's the problem with v78? The only thing I can think to do is uninstall and reinstall from scratch. The emails will be read from the server and I only use gmail calendars. Is there any way I can save the Address book?

Please don't leave me with Outlook!

For me, the solution / workaround was in my ESet anti-virus, mail-scanner, etc software. This was not a problem in TB Version 68, and only started when I updated to 78. My ESet software had not been updated / changed, this is the result of a change in Thunderbird V78. ESet support knew of the problem and showed me a setting in my ESet security / antivirus software I could change to fix the problem:

PROBLEM: Thunderbird would connect to my imap.gmail.com server, but then would spin/hang there, not downloading new emails.

CAUSE: Something (not exactly sure how to explain it) changed in the way ESet did SSL/TLS Protocol Filtering when talking with Thunderbird V78. NOTE - this did NOT happen with Firefox Version 68, it started with version 78, and is still a problem in 78.5.1 (which also changed from 64 bit to 32 bit for some reason, but thats another issue).

SOLUTION: Select ESet "Setup" (with the little gear icon on the left),

 -select Advanced Settings, (in lower right)
 -select "Web and Email" from menu on left
 - select the "+ SSL/TLS"
 - Disable first option - "Disable SSL/TLS protocol filtering"

Here is link ESet folks sent me to guide me thru it: https://support.eset.com/en/kb7728-unable-to-access-or-receive-emails-in-thunderbird-with-eset-product-installed

If you dont have ESet, but have another antivirus package, check to see if they have a similar SSL/TLS filter you can disable for email...

Since getting email worked fine in V68, but not V78 it made sense the fix should be made to/in TB. I first I tried following the advice to change Thunderbird "security.tls.version.min" from a 3 to a 1, and that seemed to solve the mail download issue, but oddly broke Firefox - every tab failed trying to establish https connection... doesnt seem logical, and I didn't spend time testing that, but thats what I observed. So I put TB security setting back to 3, and disabled SSL/TLS filter in ESet as described above. All is working now.

without specifics from users , suggestions need lots of alternative cases+infos.

there should be a warning for users in this site, before posting a message, to remind them to add TB version (if they can) , etc into the message (question/answer/response).

• reduce "security.tls.version.min" from 3 to 1, & restart TB & try again.
• disable your AV/FW/SS "mail-protection" portion only (do not disable entire/ALL function of your AV/FW/SS ), & also add+allow (POP3/IMAP, SMTP) mail-server addresses in EXCEPTION LIST so it can connect w/mail-srvrs with AV/FW/SS becomes a middle-man proxy/gateway . try again TB.
• if your ISP/MSP needs Secure-Mail-Key (SMK) for 3rd-party app, then generate a SMK code in their web-access , use that w/ TB , usually SMK codes are for one app . change password of mail account : from web-access password, into SMK code . try again TB.
• for imap use port-993, SSL/TLS . for pop use port-995 . ISP/MSP has two sets of server-adrses : one for imap+smtp , another for pop3+smtp . for-example: if you use imap for inbound then use paired smtp mail server address for outbound . some server may use STARTTLS instead of SSL/TLS for smtp , so restart TB after changes & try TB again.
• if your ISP/MSP needs 3rd-party app approved into their PAL (Permitted App List) first , then do that . ISP/MSP will send new verification code, after you try to access mails from TB, complete verification, then TB will begin to function.
• those who uses OAuth2 auth-method based mail access , sometime, after upgrading TB , you may need to (delete isp/msp's cookie) and again do the OAuth2 auth to set a new cookie.

AT&T-Yahoo (POP3/IMAP, SMTP, SMK, PAL, AV/FW/SS, etc) : here. Regain Data : downgrade, or, second-TB, or, transfer data, etc : here.