Firefox intends to keep many of the powerful feature in its version of the extension API. The previous API was powerful because it let addons reach in to the guts of Firefox's UI and tweak whatever they wanted. This meant that some parts of the browser were frozen from being changed, and extensions broke every release. There was no public-private distinction. It seems like the plan is to add most of the capabilities that were used by the old addons in the new API, so most of the useful Firefox-only addons should still work.