One of the students over at Seneca's great open source classes is considering undertaking the task of implementing a Firefox feature that would notify users if plug-ins cause Firefox to hang. What an awesome project.
I'm not a Mozilla developer, so all I can offer is words of encouragement, so, good luck, Jesse and I sure hope you go with the plug-in watcher project. That would be a huge win for the 120+ million Firefox users out there. Be a hero ;-)