I have an extremely similar, but apparently not the same, problem. Basically, I have the same symptoms as you, only instead of the "2 second" thing, I can't seek at all. It just sends the time cursor back to where it froze. Also, it usually freezes just the video and Firefox remains fine unless i close the tab or navigate away from the page. Unlike the problem here, Internet Explorer is also affected, and I've tried all the fixes I've seen here, and none seem to work, though I may be doing them wrong.
Temporary solution something that worked for me... 1. Goto tools -> add-ons -> plugins 2. disable flash plugin 3. refresh youtube webpage 4. go back to tools -> add-ons -> plugins and enable flash plugin 5. refresh the youtube webpage....tada!
I tried the fix with uninstalling adobe flash player 9 and installing 10. Happy to report it works 100%
