Probably an Adobe update.
Firefox is not a 64 bit program, there is no 64 bit Firefox available., your version woud only be 32 bit . You could try the latest version of Firefox, thatmight help the issue.
It could also be the video you are watching, it might not be compatible with the Youtube, maybe...
What is your operating system?Windows 7 x64?
How much ram do you have, you need at least 4gb.