this problem plagues me, I even saw it today, and I'm up-to-date on the latest Firefox for Fedora, I wonder if it's rolled out yet? Here's the thing, though, they probably didn't fix it for me: I use focus-follows-mouse, so probably in my case Firefox will think it has the focus (because it does briefly), but it's just a little scrap peaking out from under a stack of other windows.