Devs often also have disproportionate sway over the technical choices of friends and family, which is where Firefox got much of its footing earlier on.
Firefox has no compelling reason to exist for the average user unless privacy is a really high priority for you.
Meanwhile, devs were still targeting IE because it was where the users were. It was totally preferable to develop for Chrome or Firefox, but I recall spending countless hours still getting that stuff to IE, because that's what customers were using.
Sure, developers helped speed along adoption, but I think the new browsers ultimately succeeded because they're better products for the customer.
That, in a large part, was on web developers, making sure the user who took the leap had a good time on arrival.