So I came into this problem recently, where any youtube video that is embedded/linked from youtube will not play until I view it on its respective youtube link, using Firefox 17, I've tried reverting to the stable release of FF16, I've tried updating Java/Flash etc and nothing is working. This problem is Firefox specific, I know I could switch to another browser but I'd prefer not to if it's avoidable. This is happening on every site except youtube itself.
Edit: Just tested Chrome, and its working fine.