Symantec need to correct a mistake they made 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

Thank you! How long to these fixes normally take, days, weeks, months? In the meantime I will use IE more, but Firefox is still my first choice.

I am an ex-Symantec user partly due to problems with Symantec updates. They have a history of being slow with their updates. They have said in their support forums that the updates for 4.0.1 will be released in a few days.