What linux distribution are you using?? The normal directory for firefox is /usr/lib/firefox, but you can verify this by invoking:
in a terminal. Next you need to find where real play is installed so do
. Now you need to copy nphelix.so into the mozilla plugins directory so login in as root (su - or sudo depending on your distro) and do:
cp /path/to/nphelix /usr/lib/firefox/plugins/
Where the /path/to is the path to the realplayer plugin nphelix. Do similar for the nphelix.xpt file, except copy it into /usr/lib/firefox/components/. The symbolic link to Realplayer should be in in your path. If you can start it with realplay in a terminal then it is. This should help.