I also have troubles with this. The worst thing is that firefox often after such update dies, without saving the tabs etc. as it does when it is killed. And because it dies after update anyway, I would just suggest to put a "killall firefox" in PREIN script. Because this will trigger the firefox saving mechanism and would not cause a data loss.
Regarding arguments - you should know as you are updating - this is not true, if you are using e.g. updatesd and set it to update automaticaly.
Adam Pribyl