My primary issue with Firefox on Android is that in portrait mode, the URL bar has enough width for about five characters -- the rest being taken up by the back / forward buttons to the left, a magnifying glass icon, the QR code icon, a microphone icon, the refresh button, the tabs button, and the "three-dots" options menu. Typing a URL into a text-field that can display five characters at a time borders on ludicrous, and I have no idea how anyone thought that was reasonable to ship that as the default UI.
Other than that, I'm loving the mobile Firefox.