One of my sites uses HTML 5 to show video on the homepage. The VfE embed worked fine in Safari/Chrome/IE but had stopped working in Firefox. It just left the loading spinner spinning indefinitely and refused to play. I tracked it down to the source=".../video.ogg" line.

I entered the URL of the .ogg file directly to test it was accessible. Apache refused to serve the .ogg file, returning a "file not found" error. When I renamed it to .ogv, Firefox popped a download box which allowed me to save it locally.

And yet the spinner kept on spinning! Eventually I worked out that Apache was missing a couple of MIME type directives. I added:

AddType video/ogg .ogv
AddType application/ogg .ogg

to my .htaccess. Once I reloaded the page, the video loaded and the spinner finally stopped spinning. Best of all, when I clicked the play button it played!

I can't pin down whether this problem emerged when I changed server (and upgraded from Apache + mod_php to ZendServer Community Edition), or upgraded (progressively) from Firefox 3.5 (tested when originally embedded) to Firefox 8. Either way, it's working now.