How to disable HTML5 video on YouTube with Firefox?

I'm running Firefox (12.0 beta), and whenever a video is available in HTML5, it will open in that. I have opted-out of the HTML5 preview (I saw a post suggesting to opt-in then opt-out again, which I did... this had no effect). I want to force it to use the Flash Player always.

EDIT: I'm using Windows 7.

Thanks!

In the comments for this enhancement request there is a suggestion (comment 5 at 2012-03-13 15:57:41) to set media.webm.enabled, media.ogg.enabled and media.wave.enabled to 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.webm.enabled and 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.

Download an add-on called YouTube Flash Player. It will automatically force YouTube to use the Flash player, not HTML5.