Doesn't the kernel have an inotify system that can efficiently pass file change events back to an application (or firefox plugin in this case) without having to poll continuously?
http://en.wikipedia.org/wiki/Inotify
Possibly the same applet that warns of a kernel/system change requiring a reboot could be broadened in usefulness to give the user a specific notifier after an update that the rug might have been pulled out from under their feet for certain applications.
/Mike