You may remember the problems we had with Firefox 3.0.0 and the encoding in XMLHttpRequest back in June. The Firefox team now tried to fix it (Issue 431701 in their Bugzilla), but failed miserably, they made the problem worse (unintentially, of course :)). While it was technically correct before, now it's not even that, as it still sends ISO-8859-1 encoded content, while it claims it's UTF-8 under some circumstances. The bug reports claim, that it only happens on non-US versions of firefox, but that doesn't help us much. This new regression is tracked at #464958, is already fixed and should be in Firefox 3.0.5, which is due around mid-December. With the auto-update feature of Firefox, a lot of our users will have updated to 3.0.4 and can't really wait until mid-December, so we had to find a workaround...
In Firefox 3.0.0 there is a "strange" regression issue regarding the encoding of XMLHttpRequest requests. It's not a bug per se, it's just different behavior, which we ran into (and no other browser does it this way)