@nolok got it right, plugin stability is incredibly important to users. It's also demoralising as a plugin developer if you have to spend most of your development time testing compatibility. Mozilla need to find a balance between a fast upgrade cycle and maintaing stability for plugins. At the moment I think things have moved to far from the glacial 2.x -> 3.0 to the flash flood like 4+ upgrades. Maybe it just needs adjusting a little.