Yeah, they basically switched off a bunch of stuff in the full Mozilla suite (now Seamonkey) but it was only later (for certain things much later) that they actually removed unused code and focused on speed and memory.
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.