It's endearing that for each comment presenting a criticism of some aspect of Firefox, there's at least one reply from somebody saying, "But /I/ don't have that problem" (see thaumaturgy's top-ranked post).
So great. You don't run into problem x or problem y.
But a LOT of people do. And when I sit down at somebody's computer, and they use Firefox, and they have more than a couple tabs open, it's slow. Inconceivably slow. Especially in light of how snappy Chrome manages to be with a big stack of tabs open.
And then on a modern computer, a nice shiny new computer, things are pausing erratically, and there's lag, and it's slow, and it feels like 2005.
I can hear you leaping to Firefox's defense now: "Sure, but that was only true until 5.323, when they fixed the 'FF is slow as shit' bug!" Or, "yes, but that has never happened to /me/, and I run Firefox on a Pentium I that I spilled a bunch of grape juice on and kick every day."
Congrats, you're either an anomaly, incredibly fortunate, or you limit your number of tabs, restart Firefox regularly, and clear out your history daily. All things that I can assure you, ordinary, non-technical people NEVER DO.
Chrome managed to make a browser that doesn't become unbearably slow under normal usage patterns. Firefox, for all its moral superiority (and I gladly concede that point), has never managed to do that.