Opera 5.05 through Opera 8.x for Linux required you to have Motif (libXm.so.2) installed on your system if you wanted Netscape plug-in support:
http://www.opera.com/linux/docs/plugins/install/motif/
As of Opera 9.x, Motif is not necessary to run Opera on Linux.
Need help? Hit F1 anytime while using Opera to access our online help files, or go here.