TenFourFox doesn't work with plugins so that's not the problem. (I think the only extension I had installed was Adblock Plus.)

Maybe someday I'll wipe the crufty Tiger install off it and see if it works better after a good cleaning but perhaps it's just a matter of perception. As it is it seemed like anything compressed with H.264 is pretty much hopeless at resolutions greater than VGA even if played directly with Quicktime.