Frankly: speed. As a web developer it notice it all the time. Chrome (using Canary) is faster in every aspect; Firefox renders slowly, becomes unresponsive over time, hangs often when using web sockets (webpack); the developers tools seem to have problems with large files and so on.

I actually love Firefox' dev tools and I really would like to use it - and it's not a bad browser at all- but as a developer all the small things add up (same reason why I cannot use Safari as my daily browser).