When I installed the newest flash player I notice that some videos on youtube would play while some would not. I tried all kinds of stuff like uninstalling and reinstalling everything concerning firefox and the flash player. So after much frustration and having to use chrome I did a search and what I found out was a surprising easy fix.
The conflict is between realplayer and firefox. You know when you go to youtube to watch a video and realplayer gives you that option to download the video. Well it's that option that's the problem you have to disable that and everything will work. Even my pages load faster since I did this. Here's a video on what you need to do.