I found out today that the reason why YouTube videos were stalling after a few seconds in Firefox was because of Java. I disabled Java within Firefox, restarted Firefox, and YouTube videos are now playing without any problem. I don’t use Java much with the websites I visit so it’s no big loss. Some more information and feedback is at this link:
http://support.mozilla.com/tiki-view_forum_thread.php?forumId=1&comments_parentId=47141