For a long time I've been annoyed by really jerky playback on Flash videos under Firefox on MacOSX.
This YouTube video for example, was just awful to watch, stuttering very frequently.
Turns out the fix is pretty simple: just go into about:config and increase the
browser.sessionstore.interval setting in Firefox from the default of 10000 (10 seconds) to something larger like 120000 (120 seconds).
Got it from this page, even though it's talking about Ubuntu Firefox, it still applies to MacOSX and seems to have made a world of difference.