You have it backwards - Norton controls the compatibility of their Firefox extensions. Norton has stated repeatedly that they don't support Beta version web browsers, and Firefox 5.0 is at the Beta 5 stage right now. The actual release of Firefox 5.0 is scheduled for June 21, 2011. http://community.norton.com/t5/Norton-Internet-Security-Norton/Norton-Toolbar-not-compatible-with-FF-4-0-1/td-p/442788 http://community.norton.com/t5/Norton-Internet-Security-Norton/Firefox-4-compatibility-hotfix-is-now-available/td-p/428894

You can try Web of Trust instead: http://www.mywot.com/ Seems to be compatible with Firefox 5