The most popular websites today are all recommending that you use responsive design when developing and now the folks at Mozilla have added tools to FireFox that will help you do just that. Responsive web design is a fancy way of using fluid grids in your development. This technique allows your website to instantly adapt to a host of different screen sizes thus providing a better viewing user experience for the plethora of smart devices hitting it. The Responsive Mode will also do things like allow rotation and orientation of the screen so that the viewer will be able to check out what your site looks like landscape mode. Responsive Mode will also include several standard screen sizes in a easy to use drop down menu. Read more about this great new development tool.