I'm going to wager that it's because Firefox has devolved into a Chrome wannabe that you suffer through for ideological reasons and little else. XUL addons were

I'm going to wager that it's because Firefox has devolved into a Chrome wannabe that you suffer through for ideological reasons and little else. XUL addons were the reason to use Firefox; it's an otherwise mediocre browser that gets trounced by Chrome in every conceivable way but privacy (and even that might not be a guarantee). Mozilla started to get really into the whole ethics and responsibility arena around the same time they deprecated XUL, and them trying to act like they could do anything after giving their core competency a death sentence left a bad taste in a lot of people's mouths, mine included. I say this all as somebody who has used Firefox for almost my entire life, and will continue to use Waterfox until the wheels fall off the bus. The best thing that can happen to Firefox now is for it dwindle into irrelevance and disappear so that people can see that good intentions don't make up choosing to create the inferior product.