If you’re going to be developing a responsive website then you will need to be able to test how the site appears at different device sizes. You can simulate the view ports of a variety of devices using Firebug. How many different devices you cater for is up to you. You can use Google Analytics to find out who’s visiting and what browser they’re using. But let’s assume for the moment you want to start with: iPhone 3, iPhone 4, iPad, laptop and widescreen desktop. That’s enough to get started.
Open Firefox and choose