Error Console: [Exception... "Component returned failure code: 0x80520001 (NS_ERROR_FILE_UNRECOGNIZED_PATH) [nsIXPCComponents_Utils.readUTF8URI]" nsresult: "0x80520001 (NS_ERROR_FILE_UNRECOGNIZED_PATH)" location: "JS frame :: resource://gre/modules/L10nRegistry.jsm :: L10nRegistry.loadSync :: line 658" data: no]

I have a very similar problem: Thunderbird 78.4 and Windows 10 seeming all up to date. I have Thunderbird set up through Fasthosts with a manual configuration and POP3. Since seeing the new visual layout and therefore the 78.4 update - I have not been able to receive emails for two days. I have the same configuration set up through Outlook on a laptop and everything still works perfectly. It would therefore seem that my problem is specifically the Thunderbird 78.4 upgrade and logically the POP3 Mail Server ?

Having now contacted Fasthosts - I understand that under Thunderbird 78.4 - TLS 1.0 and TLS 1.1 has been disabled, but Fasthosts have this ‘work around’.

Basically the 'work around' is resetting the TLS back to the previous level: Thunderbird has changed the minimum level of TLS to use TLSv1.2, however the livemail server is not using the most up to date version. There is no update yet when to upgrade SSL and TLS versions on their mail server. This is what you need to do in Thunderbird to reset the minimum setting.

   In Menu app icon > Options > General
   Scroll forever to the bottom and click on 'Config Editor'
   accept the risk
   In search type: tls.version
   look for this line : security.tls.version.min
   Double click on line to open an edit window and enter the number 1
   click on OK.
   Exit window - top right x 

This will set that preference back to the same level as used automatically in version 68*.

Exit and restart Thunderbird.

I cannot access or send mail using Att.net have deleted and set up the accounts over and over again. When will these issues be resolved?