strange, the same kind of problem happend to me recently.
I use Firefox 12.0, and I access Google Groups. They (Google Groups) are changing the user interface, and the new interface is not compatible with older browsers, but it's compatible with Firefox 12.
But yesterday, when I accessed the site, it showed the old GUI instead and returned me a warning that i did not have a compatible browser, even though I had used the new GUI normaly before.
Today when I accessed the site, I tried switching back to the new GUI and it is working flawlessly again... weird, isn't it?