Long time lurker here, but I've never found a fix to the often-reported problem of the Norton toolbar disappearing in Firefox. I've had that problem intermittently on one computer, but never on my other computer. Both are running XP SP3. After the toolbar disappeared again with the recent NIS 17.5 update, I got to thinking about the difference between my two machines. Here's what I found: On the computer with the disappearing Norton toolbar, I've moved my Firefox profile folder out of it's default location (the Documents and Settings folder on the OS drive) onto a separate partition. I do this so I can easily back up the profile. This is easy to do - just edit your profiles,ini file to point it to the new location. Mozilla tells how to do this on their site, and I've done it since Firefox 1.0 and never had a problem. On my second computer that's never lost the toolbar in Firefox, I just leave the profile folder in it's default location.
Like I said, the toolbar disappeared again with the recent NIS 17.5 release. To test this, I uninstalled Firefox, uninstalled NIS, and rebooted. Then re-installed Firefox 3.5.7, leaving the profile folder untouched, and reinstalled NIS 2010/17.5. Voila! The toolbar was back!