The other biggest factor IMO Chrome greatly focusing on developer tools --> eventually stealing away firebug dev, and in turn taking the throne as the developer browser of choice.
Chrome was already very popular by the time it became any good (compared to Firefox).
But even sooner than they investing in dev-tools, they made a killing from tab isolation right at a time when Firefox had most of its stability problems.
The other biggest factor IMO Chrome greatly focusing on developer tools --> eventually stealing away firebug dev, and in turn taking the throne as the developer browser of choice.