It was announced at the Firefix Plus summit today that Firefox will include native Theora and Vorbis support for the HTML 5 media elements. So <video> and <audio> will support those codecs built into Firefox itself. Chris Blizzard posted about this earlier.
The backend has been committed to the main Mozilla source code and is enabled by default. You can download nightly builds and test it out. An example of a live site that uses <video> is the Wikimedia video archive.
This original commit is a work in progress. There are unimplemented bits, bugs, etc that need to be sorted out. But it's a start towards using a common codec across all platforms and will improve as we get towards the 3.1 release.
In other news, getting out of Whistler, where the summit is being held, is somewhat of an issue at the moment...