I never thought they did such a great job of that anyway. For example, the early versions of Firefox were about twice as big as Opera (in download size) and had a fraction of the features.
So anyone who thinks that Firefox had "Features", "Speed" or "Lightweight" as original killer features is deluded.
They did on the other hand have a fairly stripped down and therefore non-threatening usable UI (now leapfrogged in that regard by Chrome and Safari), extensions, being both open source and free of cost (without being ad supported), faking native Windows UI reasonably well, and not being IE6 in their favor.