The extension API changed (web extensions). The plugin API is still NPAPI. There was a project to replace the API with the one that chrome uses, which has better sandboxing, but eventually they abandoned that project.
My memory may be faulty, but my recollection is there was never a serious attempt to replace NPAPI with Pepper. It was looked into, but it was concluded that the Pepper APIs were too tightly bound to Webkit's implementation to make it possible to implement, and Mozilla instead built their own out-of-process sandbox (which was sorely needed because Flash crashes were the single largest cause of Firefox crashes).
