.mp4 support extension source code

For my website I currently use .flv videos. I want to switch to .mkv videos to show on my website but I don't believe firefox has support for mkv videos, only chrome. I want to create an extension that allows for the viewing of .mkv videos. I know firefox supports .mp4 videos and I would like a view of that source code so that it would be far more easy to incorporate support for .mkv videos. Is there a place where I can get .mp4 support source code so that I can write an extension for .mkv videos? Thanks,