In the comments for this enhancement request there is a suggestion (comment 5 at 2012-03-13 15:57:41) to set
false in order to "render
<video> and '' elements unable to play anything."
All these options are readily available if you enter
about:config in the address box, and then enter
media. in the filter box. In my testing (Firefox 10.0.1 on Gentoo) it was not necessary to restart Firefox, the elements were disabled when I reloaded the page.
This page was useful for my tests. The video there is disabled when both
media.ogg.enabled are set to false, as there is fallback from one to the other.
There were rumors reently that future Firefox will support h.264, in that case you might have to disable that too after an update.