Probably need to set your LIBPATH variable to point to where gecko-sdk is. Since that file is probably there. I'm surprised it did not work with xulrunner-devel install as that usually just works.
I think you need to use an absolute path to gecko-sdk when you compile it. Also you may want to see if your distro provides a xulrunner-devel package. So you don't have to use the gecko-sdk option.
Another option is to remove mplayerplug-in and install gecko-mediaplayer and gnome-mplayer
http://kdekorte.blogspot.com/2007/02/mplayerplug-in-being-changed-to-gecko.html.
Chuck,
Before you get too far with mplayerplug-in, please note that I have pretty much stopped development on it for at least 2 years. I have replaced it with gecko-mediaplayer + gnome-mplayer. Please consider looking at those before you make any changes to mplayerplug-in. gecko-mediaplayer also uses the newer npruntime api that is recommended by mozilla.
As for the socket, you should not...
