How to play HD - High Definition HTML5 videos in Firefox & Iceweasel
Install this addon:
https://addons.mozilla.org/en-US/firefox/addon/youtube-all-html5/?src=api
Goto about:config
media.mediasource.enabled
make it true.
Now you can play 720 hd html5 videos in iceweasel automatically after choosing the 720 HD option in addon and while the video is playing on youtube you can even change it to 1080 HD html5 without the need to install flash plugin. You can also disable flash plugin or block it using flashblock addon.
Thanks.
Best HTML5 addons
I no longer use those addons mentioned above.
I now use:-
http://flashblock.mozdev.org/http://www.chromeactions.com/