Firefox 5.0 is not just a bugfix release, there's new features like CSS3 animations and under the hood changes that improves performance, for example it has libjpeg-turbo which is double the performance of the older libjpeg library.
As for enterprise custom apps, all I can say is Google automatically updates all their employees to the latest Chrome when released and none of their internal apps breaks. Enterprise custom apps are really lousy if they can't even work outside of a specific browser version. Stop targeting browser versions and write apps that adhere to web standards & use feature detection.
Also a lot of enterprises uses Apple iPhone and iPads these days and I don't see anything breaking with those after they're updated with the latest iOS software update, which is almost necessary since Apple has lousy security track record. Apple isn't the most enterprise friendly company either and their focus is clearly on consumer products but it doesn't stop many enterprises from adopting iPhones and iPads.
Also there's enterprises and businesses that are adopting Chrome OS and they don't seem to have any issues with their Chrome OS notebooks automatically updating themselves to the latest Chrome OS versions.
(350k employees around the world)