How many of Firefox's UI changes really impacted this, though? There was the big move away from menu bars and to a more minimalist UI, OK, great. But all the other changes since just reshuffled things that have no net impact, other than forcing users to re-train their workflows.
Though the bigger problem is all the other crap that doesn't relate to either the UI or browsing experience. Pocket, Hello, VPN, intrusive and forced extension-based advertisements, "Studies", anti-LGBT marketing, a failed mobile OS (justified, it was just awful), some vague IoT experiments nobody knew what the point of was, …
Mozilla has half a billion dollars in annual revenue, yet somehow most of it doesn't go to either Firefox nor Thunderbird. XULRunner could've been Electron 10 years before Electron, but Mozilla didn't even care.