I'm also seeing strange behavior with SeaMonkey and YouTube. If I go to the main YouTube page (
www.youtube.com) everything looks OK, click on any video, the video page comes up with all comments and right side links like normal, but the video has a message: 'An error occurred. Please try again later.' and won't play. Then hit the 'reload' icon, get the video window, the YouTube search bar at the top and nothing else, but a 'waiting for...' message in the status bar. Now if I type 'Warpstock' in the YouTube search bar, I get a page with a bunch of videos from various Warpstocks and all of them play! So my guess is that videos with advertisements are somehow exposing a problem in SeaMonkey (since the Warpstock videos don't have any - somewhat unusual). Maybe can't handle some kind of network re-direction?
Similar situation with Firefox, but after installing the H264ify add-on in Firefox none of these problems exist. When I get some time, I'll run Wireshark with and without H264ify and see if anything interesting crops up....