No reboot has ever been needed on my current XP box. I just have to wait maybe 15 seconds before trying to restart Firefox.
Anyway, as I said in another post, it appears that the problem went away with the latest update of Firefox.
That's normally indicative of a program that has a lot of its code/data swapped out. It could very well also be that you have the setting checked to clear your cache/history on shutdown. You probably notice a fair amount of disk activity when it's shutting down because it's busy cleaning up after itself.
I've had a couple of very large programs that take a while to shut down on my box because they really have to do a lot of work before everything fully unloads. I could probably speed things up by enabling lazy writes, but I prefer not to do that.