A couple of weeks ago I moved Firefox to one side. Window disappeared, but Firefox was still running "somewhere" on my desktop, but was not actually be rendered to the screen. Killing the process and relaunching just resulted in it be rendered to this weird black hole. Log out of gnome and log back in? Same! Reboot? Same!
Ended up deleting it's config folder and re-attaching to Firefox sync in order to have it working again. No idea what went wrong, nor will I ever most likely.