We're steadily working our way towards a feature-complete implementation of XForms 1.0. We still have some work to do, and are not ready for production use, but please try our preview releases. As there are many very useful features from XForms 1.1 that makes an XForms author's life much easier, we've started to include some of those features, too. The status reported here is based off of the XForms 1.1 spec since that is, itself, based off of the XForms 1.0 spec.
We keep documentation of our current limitations and extensions to the XForms 1.0 specification on the Mozilla Developer Center