They rewrote the Firefox core, which was definitely good for performance and stability. The change also coincided with the deprecation of XUL addons, which removed a lot of desktop addons as well.

Ever since the rewrite, they’ve kept a small whitelist of addons that usually mostly work. You can make your own list of addons in the settings if you run the beta/nightly version. A lot of addons don’t work, though, because the UI elements that they interact with aren’t available (yet) on mobile.