Disable '''browser.taskbar.previews.enable '''in '''about:config''' I had this same issue, somehow I had enabled it by accident or maybe intentionally, either way I wanted to be rid of it and this does the trick. <i>'''WARNING: Changing preferences through this interface not officially supported''' Hidden settings edited using the about:config tool are explicitly not supported, which means that Mozilla makes no guarantees they will be supported in the future, or that Mozilla will fix them if they break. Mozilla does not test these preferences, and will not in the future. That includes security and performance testing which these preferences may affect. </i> ''[Warning added by moderator.]''