Why should a user program require an updater that needs to run with the root privileges? If it were on a #nix box that is locked down comparably to Windows/UAC, Firefox would've been sitting in ~/bin together with all other stuff that the user installs for himself. Windows remains the only mainstream OS that does not even have a notion of user-installed software and mandates (through its guidelines) that all software to be installed into %ProgramFiles%. There is an UAC, but there is no %UserProgramFiles%. This is truly idiotic, if you pardon my late night French, but not a bit surprising.
If you're on a Linux machine the vast majority of people are going to use a package manager to install their software. It ends up in /usr not /home.
I don't get your argument. Sure, you ~can~ install to ~/bin (and I'd guess you could change the FF install path to something outside of UAC protection/virtual folder redirection, maybe even now), but the majority doesn't. Not on Windows, not on Linux. Or are you going to tell me that the gazillion Ubuntu users install FF to ~/bin? How are they updating their packages?