This should now work in nearly all browsers, including Firefox in Windows 7 and Linux (but not Mac yet).
Example #1: Video with MP4 Source First, Ogg Second
Firefox and Opera should be able to skip the first source and play the second.
Example #2: Video with Ogg Source First, MP4 Second
Safari should be able to skip the first source and play the second.
Example #3: Video with FLV Source First, MP4 Second, Ogg Third
FLV is not supported in HTML5. However, if using HTML5 to embed the JW Player, this would be a desirable setup, and I would hope that browsers would skip the FLV source and go to a source they could support.
Example #4: Video with MP4 First, Ogg Second, using Relative @src Attributes