AdBlock and
Adblock Plus are two different products produced by two different companies.
As it says on the AdBlock web site (bolding mine):
What's in a name?
AdBlock was inspired by an extension created long ago for the older Firefox web browser called "Adblock Plus" (which was in turn named after a yet older Firefox "Adblock" project; yes, it's confusing), but it's not related to those. You've come to the right place to.