I am not here to rant about Firefox, but maybe to provide some help for those who like me have troubles with their website running with Firefox.
I don't really know if the culprit is Firefox or .Net. One thing I noticed was that the Developers.ie new design was not working properly with Firefox; all elements all over the place.
It seems that the root of the problem is the Asp.Net tag asp:panel. Firefox is considered by .Net as a downlevel browser. So I am not sure if it's because of Firefox new signature or because Firefox didn't exist at the launch of .Net.
Anyway the solution exist in .Net and seems to be quite undocumented. You have to add a Browsercaps section in your web.config or machine.config and voila it works.
I am sure that it's not perfect for every new browser, but in my opinion, I don't have time to bother with few exotic applications people may use. But of course Firefox compatibility is something important.
You can find all the details and the code here on this very useful page: http://slingfive.com/pages/code/browserCaps/
Also let me know if you have still some issues with Developers.ie and Firefox.