In the original description in Mozilla's bugzilla: https://bugzilla.mozilla.org/show_bug.cgi?id=692759, users report that the WebKit example plugin doesn't not run properly in Gecko. This is because the Core Animation drawing model is not supported since we don't use Core Animation for compositing layers. Instead Gecko uses InvalidatingCoreAnimation that is exactly the CoreAnimation drawing model with the additional requirement that NPN_InvalidateRect be called for the browser is work efficiently. See https://wiki.mozilla.org/NPAPI:InvalidatingCoreAnimation for details. The only requirement for the test plug-in is to prefer InvalidatingCoreAnimation if available and to call NPN_InvalidateRect if InvalidatingCoreAnimation is selected. See sample is here: http://svn.webkit.org/repository/webkit/trunk/Examples/NetscapeCoreAnimationPlugin/
NPAPI support is removed from Safari 14 onward and it is not supported in Webkit Builds like WebkitGTK as well. I think this can be marked as "RESOLVED WONTFIX". Thanks!
Yup, won't fix.