I don't know much about the Firefox extentions.. but I'm sure you've heard of
the greasemonkey extension?
If not, it basically lets you run bits of javascript after a page is done
loading.
And I use a script called "BlockFlash2", what it does is it takes the flash
video, hides it, and replaces it with a orange bar that says "[Play Flash]".
And when you click the orange bar, obviously, it plays the flash video.
I've always thought that the extensions for Firefox were pretty close to
javascript, so I just thought this might help you.
Here's a link to that script's source.
http://userscripts.org/scripts/review/12617