Mozilla has been working with Google on this for some time now, but we haven't been able to talk about it in public until now.* Some of the developers from the Theora project are also working with Mozilla and Google to implement and improve VP8. Firefox builds are available today:

http://nightly.mozilla.org/webm/

One of the most important parts here is that Google owns the patent rights to VP8, and is licensing them to the public under a royalty-free grant -- but the grant doesn't apply to anyone who files a patent suit against VP8, which means the patents are still useful for defensive purposes.

* This is, of course, part of the outcome Mozilla hoped for from not supporting the non-free H.264 format.