Here is a link to my test page:
All that I did was change the path to all of the support files in the html code and the graphics in the playlist css file.
The file in question is the Home Movie mp4 version.
It plays after 2-3 seconds of buffering in IE 8.0, Chrome 12, iPad with iOS 4.0 & iPod Touch with iOS 3.1.3.
It never starts with Firefox 5.0. I get the same results on my laptop running windows 7 and my desktop running XP.
The file was made from a DVD with Daniusoft DVD to MP4 using the iPad Video H.264 setting. I moved the moov atom to the beginning of the file with MP4 FastStart.
In a previous test I tried ripping the video to an m4v file with Handbrake using the default iPad settings and checking the web optimized box. I have been told that checking that box places the moov atom at the beginning of the file. The test yielded the same results.
What is strange is that the problem only occurs with Firefox and only with long videos. I have tested other mp4 files that are about 60 minutes in length and created the same way. Those play fine.