If you are on verions I was just in the same situation, and simply downloaded the installer from their main webpage [1] (after backing up my data in my profiles folder[2] and closing any running instances of Thunderbird), and it simply worked! I have to say, at least on Windows, the update looks much better, feels quite refreshing! Almost makes me want to actually check all the new emails :) [1] = https://www.thunder…
It says "currently only offered as direct download", does that mean that it'll be available as an update later on?