In defense of Firefox

The UI may need to be single-threaded. But handling the events need not be.

Control could, and should, return to the UI-thread without waiting for the event to be
finished handling.

Just because tab a is currently in the process of repainting, there's no reason to -not-
process new events coming in.

Yeah, yeah, I know -why- FF is like this. But that doesn't change the fact that this
particular aspect truly sucks.