From
http://plugindoc.mozdev.org/linux.html:
1. Install RealPlayer 10.
2. Copy nphelix.so to your Mozilla plugins directory and nphelix.xpt to your Mozilla components directory.
3. Make sure a symbolic link to the realplay script is in your PATH.
What in the world is the Mozilla components direcotry? I know the plugins directory is ~/.mozilla/plugins, but I haven't seen any documentation on the latter in MozDev or on Google.