This plugin may work well when you have iTunes on your computer, but without iTunes this plugin won't work at all :-( This is probably because of the heavily proprietary character of all Apple related software.
Hi chillgreg, I just checked iTunes Search with FF 4.0 and iTunes 10.2.1 on Mac and Windows and it worked fine, found an old Windows install of 10.0.1.22, tried that with FF 4 and it worked fine too. Upgrading to 10.2.1 couldn't hurt, I'd start with that. Could you also try "Restart with Add-ons Disabled" from the Help menu and then try iTunes Search? If that works, you have an add-on installed that's interfering with things. Start Firefox normally and begin disabling add-ons one at a time, restarting, and testing the search until you've found the culprit.
Ghostery is known to cause issues with iTunes Search, there may be other add-ons that are problematic. If you find one, I'd be interested to know ;)