Mozilla has recently added two simple but very helpful aids to Firefox’s web developer toolset to ease the challenge of responsive design: designing and coding web interfaces for several displays, with several sizes, ratios, and features.
First, Responsive Mode gives you a variety of well known screen resolutions so you can simulate different screens in a landscape or portrait orientation. Just select Responsive Mode in the Firefox menu, and select the desired screen size and orientation.
Second, when you are inspecting an HTML element, common size related attributes like width, height, padding, border, and margin, are now displayed in the lower area of the Style panel. Open it, and click on the minimized widget below.
These tools should become generally available with Firefox 15, around early September. If you want to try it now, grab an Aurora 15 build today.