What is NPAPI ?
NPAPI is the legacy plugin system used by browsers that allows webpages to serve executable content without the user having to download a file.
This system is used by Flash, Unity, Java, and various unimportant plugins. Of these, Flash has an arrangement with Adobe, Unity has an exit strategy, and Java is completely neutered as it was for quite some time. The unimportant plugins are unimportant (and if they were, they'd have fixed it by now.)
and does this have anything to do with the add=ons and plug=ins specific to Firefox and Seamonkey
SAome of which break every time they put out a new version of FF
Those are extensions, which is completely different.