It's more than a user interface though. It's a platform for Web applications that let those apps reach all the way down to the full capabilities of the hardware.
Perhaps we share a definition disconnect. Is X11 a platform for a user interface or is it a OS?
I lean to the former definition but I'll be the first to agree that I come from an earlier time. When 'windows' came out it really was a user interface on top of MS/PC-DOS (and it still is IMO) however now several generations of folks have been brought up saying that Windows was the OS.
I love that the mozilla guys have done the legwork of figuring out what APIs were missing in order to do that. That is a nice piece of work.