recently i noticed thin black bar appear in bottom right corner of firefox window. then i noticed thin black bar appear in button tooltip too. this bar gone when i start firefox in safe mode, so i decided to refresh firefox to delete addons and plugins, but it still there. then i tried to uninstall firefox, download and install a fresh one, same bar appear, gone when in safe mode, i didnt add any plugin or extension yet. i'll add images later, as i post this in safe mode.