One major reason why I keep coming back to Firefox is its add-on integration with LastPass (my password vault). Chrome for Android doesn’t let LastPass autofill; and Opera for Android doesn’t have an option to disable Opera’s built-in password manager. Also, Chrome for PC annoyingly disables addons that aren’t downloaded from the Chrome Web store.
Among other reasons.
Firefox basically does what I want it to and doesn’t complain about third party addons.