&gt; One might say that Mozilla will wait to release the new Firefox till it has all the old features of the old Firefox, but that&#x27;s not been my experience with how teams work.<p>Realistically, getting to feature parity after rewriting a core part of any application is going to be nearly impossible.  You end up with different features, hopefully better ones, but not exactly the ones you had before you started.  You can&#x27;t step twice in the same river.