Following the Thunderbird 78.11.0 update on Debian 11, the application is no longer able to read Gmail mail, and possibly other mail. The application also does not allow you to configure a new Gmail email account, and perhaps other email accounts, whether automatic or manual or this message is displayed:

This problem is also materialized by this error message when launching a not yet configured Thunderbird, see the second tab:

This bug has been reported here following the Thunderbird 78.11.0 update on Debian 11. This bug is not present on Debian 10.

In order to use Thunderbird on Debian 11, while waiting for the patch, you can set the option network.http.spdy.enforce-tls-profile to false in the configuration of this application.
To do this go to Edit> Preferences> General tab and at the very bottom of the page, you must click on “Config Editor …”:

In the information window that pops up, click on “I accepted the risk !”:

Then in the search bar enter (or copy/paste) the string network.http.spdy.enforce-tls-profile and then double click on the selected line in order to change this option to “false”:

Then, close this settings window, close Thunderbird and relaunch it. The second error tab should be gone: