type this in the address bar: about:plugins
- there should be two “Shockwave Flash” plugins
- click DISABLE on the “gcswf32.dll” one near the top
- leave the “npswf32.dll” version enabled near the bottom on the page

Also you can try this :-

Options
Under the bonnet tab > Content Settings
Cookies > Exceptions
Add… and then add www.youtube.com and [*.]youtube.com