When Chrome first came out, I used Chrome as a secondary/tertiary/(whatever I didn't need that many active browsers back then) browser, with Fx as my main.
But then there was a version of Fx3 (oh god pre-rapid release, hi) that had huge memory leaking issues where I would just peg my ram after a few days.
Nowadays, when I start a computer from scratch, I pretty much only install Chrome and Seamonkey at the beginning. And add Firefox whenever I need a third browser.
But that said, Mozilla is planning on moving away from Gecko?
Makes sense. Chromium moved from WebKit to Blink, but Blink is ultimately a fork of WebKit.
Microsoft moved from Trident to EdgeHTML.
Opera just flat-out abandoned Presto for Blink.