Firefox 3.6 will be released today, Januar 21st. Unfortunately the XForms extension for this version is not yet ready. We're planning an official release for Firefox 3.6 within this quarter, see bug 539184 for details. The nightly builds for 3.6 are currently broken as well, I hope to get this fixed until the end of the week.
As a side note, there will be no official release for XForms for Firefox 3.5. The XPI package available at my builds page has been tested by many people and works well, but no further development efforts will go into that version.