This was also asked here: https://support.mozilla.com/en-US/questions/837142 Google Toolbar is an addon, and every addon has a maximum version number set by the addon developers. Google Toolbar apparently is compatible with Firefox 5, and the only reason it doesn't work is because the Google developers have not updated their toolbar's maximum version number. You can either wait for them to do so, or follow the directions at the link above to change the number yourself.