Try the suggestion on Norton's forums here: Help with FF4 Compatibility Update

Also please read this article to see if it helps:

http://community.norton.com/t5/Norton-Internet-Security-Norton/Norton-Toolbar-not-compatible-with-FF-4-0-1/td-p/442788/page/2

Read the post by "drminer".

@ silkphoenix, there's also a post from Norton's community service manager recommending not to interfere with the the product's files as it may compromise security. See the post following the one you mentioned.

Xircal, Thanks for the warning. I guess I didn't read the whole thread thoroughly.