1.4.3 - No difference compared to 1.4.2... Puntuado con 2 de 5 estrellas
...referring to using the context menu.
Meanwhile, I think I know, what's wrong.
The effect I described before seems to have something to do with the "active" option in the menu of statusbar icon.
If that option is checked, everything is ok.
If it's unchecked, the background gets stuck.
The problem is, without looking at the according menu you can't tell the checked state from the unchecked one, because the icon doesn't turn grey in TB, as it does in FF, and because translations are performed (as for 1.4.2 and above) in TB even with this "active" option unchecked.
That was difficult... But we didn't give up ;)
The "active" option is now still there, still without a function in ThunderBird, but everything else should work correctly now.
Thanks you Centauri39 for the feedback and the very good investigation!