Popcap pointed me at this: http://downloadsquad.switched.com/2009/08/22/how-to-fix-flash-video-performance-in-firefox/
Do YouTube and other Flash-based videos stutter when you view them in Firefox, but work fine in other browsers like Internet Explorer, Chrome, and Safari? This appears to be a problem that affects both the Windows and Mac versions of Firefox, and the problem (surprisingly enough) turns out not to be Flash - it's Firefox's session restore feature.
Lifehacker has a post based on information that one of their readers wrote in with. Basically, Firefox takes a snapshot of all of your open tabs every ten seconds, so that if you close your browser, you can open it to where you left off. Though you wouldn't think grabbing the plain text URL of a few webpages would be such an arduous task for the browser that it causes video to stutter, for some reason it is.
So what's the solution? Well, if you want to throw the baby out with the bath water, you can turn off the session restore feature altogether. A more sane solution is to simply lengthen the period of time between snapshots. To do this, type about:config into Firefox's address bar, then in the filter box enter browser.sessionstore.interval. The default value is 10000, which is the number of milliseconds between snapshots. Setting the number to 120000 lengthens the period to two minutes, a more reasonable but still useful period. Of course, you can set it to whatever you want, as long as you keep in mind that you're dealing with milliseconds.