I often find myself needing to reposition my Thunderbird window so I can read it and another window at the same time. Unfortunately, there doesn't seem to be an obvious place on the titlebar that's safe to click and drag. Instead I have to carefully hover my mouse around in the titlebar trying to find a spot that's not a button/action. The existing alternative is to disable the "Hide system window titlebar" in the Language and appearance settings, but that results is a fair amount of wasted screen space.
So, my suggestion is to add some kind of obvious non-active object to the title bar that can be used as a move handle. This seems like it should be easy to implement. The bigger 'design' issue would be what image or text to use. 'ThunderBird', or its icon would be the obvious choices.
Note: I see two other similar existing 'ideas' that relate to moving the window, but both seem to suggest using the 'View' tab as the handle. Unfortunately, I don't see a 'View' tab in my version of thunderbird (Windows/Supernova - 115.1.1) or I'd have just voted for one of these.