Are you using RealPlayer 10?
If not, do cus it's loads better than rp8.
Install it as root, install it to /usr/local/RealPlayer, say yes to "system wide links" and give it /usr for the last path it asks for.
Then, invoke it with
and tell it to "configure browser helpers" when it starts up.
This should sort everything out.
If not, copy the realplayer plugins to the firefox/plugins folder.