Thunderbird version 78.0 is only offered as direct download from thunderbird.net and not as an upgrade from Thunderbird version 68 or earlier. A future release will provide updates from earlier versions.
Add-on support: As of version 78.0, Thunderbird only supports MailExtensions. Your favorite add-ons may not have been updated for compatibility.
At this time, users of the Enigmail Add-on should not update to Thunderbird 78.