We found this directory up on Mozilla’s servers today with this note in place:

Welcome! This directory contains pre- pre-pre-alpha builds of Mozilla2. They are not yet representative of what Firefox 4 will look like. These builds are not regularly tested - use at your own risk. For those interested, the source code is here.
Hmm this is exactly what virtual machines are for! Let’s see what Mozillia has in store for us! Who’s going in? Feedback in the comments guys!