Another reason on top of what’s been mentioned already (although probably minor), is that out of the box, Firefox doesn’t let you run multiple instances.

I’ve been learning to write a web app and updating websites, so have been using PortableApps to launch a second instance of Chrome to double check how everything looks when I’m not logged in. I tried switching to Firefox, but it wouldn’t let me open the second instance, meaning that every time I wanted to check the site, I’d have to log out. I check them in Chrome, Firefox, and Opera.

I might be a niche case, but I’m already finding it really annoying. I can’t imagine how much more frustrating it would be to try to write a browser that can’t run at the same time as your preferred browser.