On Firefox on Linux, pressing the Mod4 key (also known as Windows key) brings up my Bookmarks (the sidebar). I've had this happen before on Windows years ago and couldn't find a solution (had to reinstall the OS back then). No setting was changed between the time it was working (1 hour ago) and the time such behavior appeared (10 minutes ago). I am constantly using the Mod4 key for window manager functions, so it's very unusable now on Firefox's window.