The Mozilla plugins folder is inside a folder called .mozilla in your home directory.
It is a hidden file.
As for the dll's I'm guessing they're generated during the RealPlayer install.
I don't use RealPlayer, I use Gxine with Firefox.
If you have Gxine installed, get the rp9 codecs from:
http://ftp3.mplayerhq.hu/MPlayer/releases/codecs/
Untar them to your home directory (or wherever) , open a terminal and
type gxine
Follow the instructions.
You can then go to the preferences in Gxine, select the codecs tab and type in
the path to the rp9codecs folder. (/home/foo/rp9codecs)
Close Gxine.
Firefox should now pick up RealPlayer streams with Gxine.