"I can install it immediately and in Linux, one usually has to upgrade their entire distribution. In Linux, there is no fundamental differentiation between system packages and even leaf applications. This is a problem that one has to solve and the current methods are awfully weak. "
How is upgrading Firefox and depenndecies "upgrading their entire distribution"? Debian will only upgrade packages that have changed or bring in new packages that are needed for the system.
Note that debian puts xulrunner in a separate package so that it is avialable for various packages.
Also, when I put iceweasel/experimental as an example - just upgradeing iceweasel to the latest version in whataver Debian distro you're using (stable, testing, unstable) is "apt-get -f install iceweasel".