Firefox, because I like it as a browser. Plus its Javascript error messages are often more illuminating than Chrome's.

Also, cross-browser compatibility is important to me, and I figure all my colleagues test stuff in Chrome and will complain about stuff that doesn't work there. I can't tell you how many Firefox-specific bugs I've found in our site.

The tough part is finding someone who will regularly check stuff out in IE or Edge, if you don't want to be spinning VMs up and down all day every day.