Just installed latest Firefox 2.0 on my Windows box. It works as expected. Can't see a technical reason why the release number was bumped to "2.0" when there are just a lot of minor changes and fixes.
Having a close button on the right hand side of each tab is more intuitive. Previously, there was only one close button on the right hand side of the Firefox window for closing the currently active tab. I had no problem with that but I noticed my son didn't realise he could close tabs by pressing that button. Score an extra point for usability.
Update: Firefox 2.0 can spell-check text fields. Now I know what rubbish I've been writing.