The difference is that very few if any plugins are sold and it is somewhat easy to downgrade to an older version of Firefox. Could you imagine what would happen if iPhone OS 2.3 came out and 10% of the apps in the itunes store stopped working? It would be a PR nightmare for Apple.
Well, another big point is that Mozilla pre-releases it's software. I'm not 100% aware, but I don't think Apple provides the same convenience to developers.
Wouldn't you like three days to see if your software is going to run smoothly after the update?
