According to this Bugzilla entry, Bug 424923 - Remove Cross-Site XHR, the Cross-Site XMLHttpRequest (XHR) support has been removed from Mozilla Firefox 3. Mike Shaver made brief mention of this in his latest blog post.
I think this is good news overall. It just didn't seem that the whole concept of cross-site XHR was fully baked. Given the prevalence of cross-domain web attacks, waiting for the specification to settle is probably an excellent idea.