Sounds like their browser detection is, um.... mis-implemented. And their redirect code is, too.
Have you tried changing your user-agent string back to 1.0.3 using the user-agent-switcher extension, or any of the other similar extensions? If it works when you do that, then it's a legitimate bug on Google's end.
The two UA strings are: Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.7.8) Gecko/20050511 Firefox/1.0.4 Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.7.7) Gecko/20050414 Firefox/1.0.3