indiWiz.com

I found this interesting link listing the most interesting Firefox plugins.

To encode a video file for use in HTML5 video compatible browser (firefox 3.5 and above supports HTML5 video), use the command:

$ ffmpeg -i in.flv -vcodec libtheora -sameq \
   -acodec libvorbis -ac 2 -sameq out.ogg

Finally use it in your page:

<video src="out.ogg">
Your browser does not support HTML5 video.
</video>

Reference: HTML 5 <video> Tag.

Have you upgraded to Firefox 3.5? Read a review of its new features.