I'm bumping this thread again because I'm seeing this issue in other Mozilla-based browsers:
Firefox 3.1 nightlies: Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.9.1a2pre) Gecko/2008073003 Minefield/3.1a2pre
K-Meleon on Windows: Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.8.1.17pre) Gecko/20080716 K-Meleon/1.1.6
Changing the user agent of either browser to that of Firefox 2 will correct the issue and everything works as expected. I'd assume that other browsers, like Camino and Seamonkey, will also see the same error, but I can't test them right now.
Please respect your users' browser choices by detecting Mozilla-based browsers correctly (by looking for Gecko/ in the User Agent string rather than Firefox, and looking at the number following rv: if you need specific features).