Please update to a modern and supported version of Firefox,Firefox 22 or 23 that comes out next week. Firefox 4 is extremely insecure and hasnt been supported for some time.

I needed to change the plugins.scan.plid.all settings in about:config. It's working fine now.

Still, why are you using Firefox 4?

I've found FF4 to be stable and have no reason to change. I'm sure there are security issues, but I have never had any problems with it. I do run my browsers sandboxed (I realize this isn't a complete security solution). I just don;t have the time, the energy or the inclination to get on the upgrrade treadmill, and have to deal with the unending series of compatibility issues that arise from doing so. Thanks for your interest.

I'd like to ask you to reconsider you decision to use Firefox 4. Using modern versions of Firefox not only protects you from hundreds of security holes, https://www.mozilla.org/security/known-vulnerabilities/firefox.html. But it also will give you a much more stable and quick web browser. For 99.99% of users who update they don;e have any issues to deal with when they upgrade. I'd suggest you try updating and see if your Firefox works, you'll be surprised by how much better it is and how easy it is to update.