The Norton for Mac troubleshooting team invested a lot of time, effort and patience in this forum and in private Norton mail, in helping me with my Safari 6.0.5 and Norton Internet Security 5.5 plug-in.
The problem, I'm delighted to report to them and share with others who use Safari and Norton, was NOT in my Norton plug-in or any of my NIS 5.5 components nor was it in Apple's Safari application or any other bug.
Although I had done a Safari re-set several times before and after I updated to the latest Mac OS 10.8 and Safari versions, I had not gone far enough in error-and-trial testing of the browser's preferences.
In removing the check mark from "Allow Java" in Safari's Security preferences, I must have also unchecked "Allow all other plug-ins." Since placing a checkmark in that box today, my Norton plug-in works perfectly!
Thanks very much for all the help!