What is HTML 5  ?
Its a new web standard for developing web sites and web applications. W3C is bringing in more advance features in HTML, so that you can develop more interactive features in your web applications and users can access them without downloading any plugins like flash player or Microsoft silverlight plugins or any such replated plugins. These features will be built in your browsers, For example, HTML5 introduced a <video> tag to show videos in your website; in current version of HTML you do it either by <object> / <embed> tags which also require you to download some plugins / activex controls for flash or silverlight.

YouTube has recently launched HTML5 version, which plays video with the HTML5 <video> control. Early you need to download a flash player to view youtube videos and with HTML5 you need not do that. This version was released since Apple launched the iPAD which do not support flash and has only HTML5 video support. So now iPad users can view YouTube HTML5 videos using HTML5 enabled safari browser.

But YouTube do not support Firefox… and what is the reason.

YouTube videos are using H.264 codec to convert their videos which is a patented technology, meaning that it is illegal to use without paying license fees. ( Google and Apple are paying a huge amount for using this codec.)

Which means that if your need to show Youtube videos using HTML5 You need to embeed this codec in browsers HTML parser.

Firefox is a free open souce product which designed with open web concept and they do not believe in using paid codecs in their browser. Firefox support open media format. (.OGG format)

Youtube using H.264 codec has brough a big hurdle for Firefox users. Though Youtube Flash player is still supported and firefox users can view youtube video.  At present youtube HTML5 vides are only supported by Google Chrome, Apple Safari and Microsoft IE with Google Chrome extension.

Hope soon firefox will come with some work around to access YouTubes HTML5 videos in Firefox.

Below is a Demo of  HTML5 video, This video is in Open Media Format ( .Ogg format )… u can view it in Chrome, FireFox, Opera, IE with Chrome extension and Safari / other HTML5 compatible browsers

Source Code :
<video src="http://videobin.org/+yz/14s.ogg" id="video" 
width="300" height="240" autoplay controls></video>