It is down to Symantec to correct their mistake with their add-ons. They originally updated their add-ons after quite a delay to work with Firefox 4.0 as shown here - http://community.norton.com/t5/Norton-Internet-Security-Norton/Firefox-4-compatibility-hotfix-is-now-available/td-p/428894 They made an error with the update in only listing it as compatible with Firefox 4.0, so they will not install on Firefox 4.0.1 and subsequent Firefox security updates. Symantec have indicated they will be releasing an update to correct their error soon, for details see http://community.norton.com/t5/Norton-Internet-Security-Norton/Norton-Toolbar-not-compatible-with-FF-4-0-1/td-p/442788/page/12