Polypane is a browser built specifically for developing responsive websites. It can present typical device resolutions side-by-side (for example iphone SE next to iphone 7 next to iPad) but also has some nice features such as automatically creating views based on your stylesheet’s media query breakpoints.

It’s a subscription service and at the moment I’m happy using a combination of Firefox Nightly and Chrome so I think I’ll wait this one out for the time being but keep my eye on it.