https://blog.mozilla.org/addons/2015/08/21/the-future-of-developing-firefox-add-ons/
Looks like I have no choice but to switch fully to Pale Moon now... SeaMonkey will be sucked down with the Firefox Titanic. So... hi everyone. I guess I'm properly on board now. And any XUL-based extensions I make in future.Deprecation of XUL, XPCOM, and the permissive add-on model
Oh and Moonchild for God's sake don't jettison features from PM for no good reason and don't do stuff "because Chrome did it".