youtube in the latest firefox doesn't display videos in Linux Mint 15 olivia

youtube in the latest firefox doesn't display videos in Linux Mint 15 olivia

I'm running Linux mint 15 olivia, Xfce 32-bit version. I try to access
youtube videos in the latest Firefox, but the videos displays incorrectly
for some reason. I do have flash installed, but I see there is more than
one flash plugin that can be installed when I go the software manager. I
installed Google Chrome also and there youtube videos displays correctly,
I can watch the videos as normal. Why is this? What can I do to fix
firefox? I can see that I have shockwave player also installed. Please
assist.