I cannot see youtube movie on firefox. How to solve it?

I cannot see youtube movie on firefox, also, my computer is very slow.
How to solve it? Please help me. Thank you.
BTW, I can see youtube movie on I/E.

OLD compter ? what's version of windows system ?
use what's kinds hardware ?

are you ready update Adobe Flash Player 12.x.xxx.xX ? ? ?
firefox add-on : Adobe Shockwave Player 11.x


IE or Chrome browser can see youtube movie ?