http://en.opensuse.org/Firefox
These aren't terribly difficult for developers to set up, certainly easier than creating an installer for Windows. I wish Mozilla, OpenOffice, etc. offered this sort of download as the default, with tarballs only for users who know what they're getting themselves into.
The main pain is presumably in generating these for different distros. Novell's build service is supposed to make this easier, but I haven't tried it:
http://en.opensuse.org/Build_Service/supported_build_targets
One more thing that would help with this is if more browsers used the distro in the user-agent string, which would help with presenting the right installer to the user. Distro-built versions of Firefox certainly include e.g. "SUSE", but not the version. Windows and Mac browsers send the OS arch and version ("Windows NT 5.2", "Intel Mac OS X 10.5") so just sending "Linux x86_64" seems silly.
Which is why Google Chrome is a potential game changer