Thunderbird doesn't have jumplists in the Icons-only task manager. It does however have jumplists in the application launcher.
Thunderbird has "OnlyShowIn=Unity" in its .desktop file. So far we only respect this restriction in task manager since I've seen players adding media controls to their menu which would clash with the ones we add. I didn't bother doing that in the launcher menus. See also a bug [1] I filed against Firefox which has the same but nobody cared. Feel free to file a separate bug report against Thunderbird over there. Thanks. [1] https://bugs.launchpad.net/ubuntu/+source/firefox/+bug/1615549
Thanks for the information. I modified the .desktop file and can now access the jumplist.
> I didn't bother doing that in the launcher menus. We really need that common lib you talked about to address these synchronization issues :( Kicker and TM menus drift more and more for similar things.