I ran into a problem this morning that had me stumped. One of our sites was running fine in IE but in Firefox was causing a “400 Bad Request”. To make matters more confusing the server was hosting two domains, one of them working in Firefox and one not. After assessing the server and ensuring everything was set-up properly I knew the problem had to be related to the browser. Sure enough, after explicitly removing all cookies from Firefox the problem was solved. Weird :S

Tools > Options > Privacy > Show Cookies > Remove All Cookies