Why I never use it. I will always stick to Firefox. Firefox has yet to irritate me. A few plugins here or there is not as awful as most of what Google does.
Besides if I REALLY wanted to I could run one of the GPL forks of Firefox. They may be dated after a while but they work usually.
Edit: I meant to say GNU forks like IceWeasel and GNU IceCat which are licensed as GPL usually. There's other forks too.
It's GPL compatible, and I wrote GPL instead of GNU. There's IceWeasel which is a Debian fork of Firefox to debrand it from all the Firefox trademarks and logos.