I use a custom window manager. I used to be able to move and resize Firefox windows in fullscreen mode (F11). This is not possible anymore now, and hasn't been since around version 55.

I would like to find out why the fullscreen mode doesn't respond to calls from my window manager anymore, and whether there might be modifiers to revert to previous behaviour, or to otherwise remedy the issue. The window manager has not changed, and is still able to control other F11-fullscreened applications, such as Windows Explorer folders and Microsoft Edge.