This is an issue being caused by an installed add-on/extension. Recently, I helped another individual solve the same problem.

Start FireFox in Safe Mode which disables all add-on/extensions & try to watch the same videos again. Then simply enable each add-on individually again until the problem resurfaces to identify the add-on causing the problem.

See: Diagnose Firefox issues using Troubleshoot Mode

I did what you suggested and I just re-enabled some of my add-ons. It seems to have worked...Thanks

Go to tools in firefox , click on add-ons , click on extensions, disable Real Player, next uninstall real player, next uninstall flash player 11.3 downgrade your flash player to 10.2 then restart Mozilla firefox and you can watch video streaming again from facebook and or directly from youtube. Hope this is helpful,, it fixed my problem after a week of scratching my head! Also you can check by typing "about:plugins" into their address bar, instead of a URL, users can see a list of all the software plug-ins that are currently working with Mozilla Firefox. If Flash is properly installed, they will see an entry for "Shockwave Flash" on the page that will come up. If this entry is not present, then Flash is not installed and the user needs to install it before he, she will be able to watch YouTube videos in Firefox.

Read more: YouTube Won't Play in Mozilla Firefox | eHow.com http://www.ehow.com/info_12158867_youtube-wont-play-mozilla-firefox.html#ixzz1yqW0O2rN

I have the same problem but you can try what i did. It work on my fire fox . Open you browser then go help and about fire fox. This will take you another website and waiting for download to completed and update your browser. My one is working after that. Or Go to help and click on troubleshooting information then click reset fire fox on the top right. It will go back to the first time you install it without loosing all your bookmarks.

Best of luck