I can play video on the youtube website, but I cannot play youtube videos embedded in facebook or other websites. Captchas are also not showing up, I suspect the two problems are really the same problem. This started about a week or two ago.
"... I already tried uninstalling and reinstalling firefox."
Uninstalling and reinstalling is mostly a waste of time for problems with your browser. You should look at these two references:
Problems with browsers commonly arise because of
To elaborate, whether an add-on or plug-in is responsible for your problem, a "safe mode" is available and its usage is detailed in the first link.
It turned out that the culprit was the Google Disconnect 2.4.2 add-on. Disabling it fixes the problem. I am using Firefox 17.0.1.