I was trying to take advantage of that new HTML5 <video> tag, and Firefox 3.5's native Ogg support, by embedding an .ogv video. Here is the references I used:
You need to make sure you your web server is set-up with the correct MIME types. Otherwise, Firefox 3.5 will not show the video.
I found the following at http://camendesign.com/code/ and it worked for me. Add the following to your .htaccess file:
AddType video/ogg .ogv
AddType video/mp4 .mp4
AddType video/x-m4v .m4v
The Camen Design site has a good overview of using the <video> tag with a Flash fallback (for browsers that don't use the <video> tag). Hope that helps.