I would guess that it's possible that the way X works prevents this from happening. At least, easily.
I use an offbeat windowing manager (WM) (aka, the client to Xorg that actually draws the base-level windows for the GUI). The WM gets to present titlebar as it pleases, and given how my WM works, there's 0 chance that Firefox would be able to draw it's own stuff up in there.
Given that, perhaps some WMs can allow the client window to draw up there and some can not. I get the impression that, at a minimum, the titlebar is not guaranteed to be fair game.
I use an offbeat windowing manager (WM) (aka, the client to Xorg that actually draws the base-level windows for the GUI). The WM gets to present titlebar as it pleases, and given how my WM works, there's 0 chance that Firefox would be able to draw it's own stuff up in there.
Given that, perhaps some WMs can allow the client window to draw up there and some can not. I get the impression that, at a minimum, the titlebar is not guaranteed to be fair game.