HTML5: If it works in IE, It might not in Firefox

It seems, as best as this humble novice html code writer can discover, that if you use mp4 video as the source, Internet Explorer might play the video. Then, all of a sudden, Firefox might not. Chrome seems to play all the formats. It all has to do with the format surely now. So, three browsers will play OGV and WebM, while IE needs it to be in mp4 format. And a lot of people still use IE. Tomorrow, I'll try to sort this out so my videos and audios play on all browsers.