After doing a little reading I found that this is not a bug, but in fact a feature. Firefox has the ability to be installed as a stand alone application without admin privileges. The user has to understand this and just change the directory to a folder in their user files. The downside to this is Firefox will only be installed for that user only unless you first run the installer as admin and install the browser to the program directory.
I'm not certain I like the approach they are taking here. Mozilla should either go ahead and have the installer create a Firefox directory in the user's folder or have it default to the programs folder and ask for elevated privilege. If they chose the second, they could create a dialog giving the user the option to install it to their own directory without privilege escalation. This is really confusing and short sighted on their part for Vista users.
Edit:
This only happens this way on a limited user account from what I can tell. You should still get a UAC prompt on an admin account.