I'm getting intermittant "400 bad header" errors. Given this error can be transient but persistent, I thought I'd let you know it's still happening if you thought it had gone away, or as a starter point if you didn't know about it
When I logon I get the 403 forbidden notice but it doesn't stop me using the site, however, when typing a reply, the keyboard seems to have a delay and miss out certain letters which means I have to keep backtracking (bloody annoying it is too)
Laptop with firefox also.
Desktop - Win 7/64 with Firefox 21. It hasn't come back for me, but 400 is one of the real hard to find and fix ones as it tends to come and go, and you may well get the condition when there isn't a request.
I would have got back to you earlier, but Mk1 was being a little sod with his homework