Increasing the size of all Flash videos in Firefox

Increasing the size of all Flash videos in Firefox

Ubuntu won't play full-screen Flash videos smoothly, and they never look very good blown up that much, anyway. They don't look very good at native resolution, either. I've got a Stylish theme for YouTube to blow up those videos to about double the native size, and I wrote a GreaseMonkey script to do the same on a few other sites, but is there a general way to do this on every site?