Recently I had the same problem as you, and I and I've solved it by
installing the addon "YouTube All HTML5"[0].
This addon automatically disables Gnash for YouTube and turn on the HTML5
player (even for videos that are not supported by YouTube).
And it's free software, is licensed under GPL/MPL.
[0] https://addons.mozilla.org/en-US/firefox/addon/youtube-all-html5/?src=api
--
Damián Muraña
Usuario de GNU/Linux #460383
http://www.murana.uy/
http://www.trisquel.info/
Use estándares abiertos:
http://documentfreedom.org/openstandards.es.html
