Firefox was a mess for a long time. It's not the speed that mattered but the stability. Chrome isolated everything into separated threads and processes. One tab

Firefox was a mess for a long time. It's not the speed that mattered but the stability. Chrome isolated everything into separated threads and processes. One tab couldn't kill the whole browser. Firefox didn't and initially even the UI was in the same thread as the webpage javascript. Doing the switch required having all extensions rewritten so they lost market share in the process (but had no choice by then).