As a heads up to developers, Cross Site XMLHttpRequest (XHR) has been removed from Firefox 3, despite being in earlier betas. To quote a draft relnote from Mike Beltzner:
Due to late changes in the Cross Site XMLHttpRequest specification which made our implementation incomplete, it was decided to remove support for this technology rather than include only partial support.
Hopefully things will be more stable by the next release and it can be re-included.