2 things keep me on Chrome. First, the overly large titlebar on Linux. I've tried some addons to make it more like Chrome but they all look like shit. And second, the dev tools. I find dev tools in Firefox clumsy to use. I believe I originally switched to Chrome for the dev tools when I ventured into frontend development.
Just installed Chromium to compare. When maximized, the Chromium title bar is 65px high on my system, whereas Firefox's takes 70px. I should note, though, that I have set my window manager to hide the default window decoration on maximized windows, which helps.