You need flash player to play. You might have flash but the current version of flash (flash 11) have various issues. You need to downgrade flash to a previous verion.
Here is what you can do: (Im assuming you have flash 11 and firefox 14.0.1):
Download this file : http://www.ziddu.com/download/20225702/libflashplayer.zip.html
Create a folder "plugins" under "/home/YOUR-USER-NAME/.mozilla/firefox/"
Like this: /home/leopard/.mozilla/firefox/plugins
Extract the zip and copy the downloaded "libflashplayer.so" to "/home/leopard/.mozilla/firefox/plugins"
Open nautilus as Admin. Go to "/usr/lib/adobe-flashplugin". Delete the "libflashplayer.so" there.
Copy the extracted "libflashplayer.so" there.