The add-on ‘Full Screen Video’ lets you view these videos in full screen mode. You can install Full Screen Video from here. When right click on any HTML 5 video you can see a ‘Full Screen menu, just click it to see in full mode.
As you know Mozilla’s new browser Firefox 3.5 comes with new web technologies. It HTML 5 support lets you view video and audio content from within the browser without installing the plug-ins. Now everyone can watch open format video and audio through Firefox. Here is an add-on for viewing HTML 5 video in full screen mode.