"Web developer" is basically a Firefox add-on, which comes in a bundle of lot of web developer tool. This supports almost anything thing you want do with web pages like edit CSS on fly, edit html, meta tag information, view hidden elements, borders, images info, forms, cookies.... and lot more. You can easily extract styles, scripts or images from other web pages. I recommend this for a web developer. This could save lots of time which actually involves change in designer code and compile and view again and again.
The best part I liked about this is edit the html on the fly and see your changes immediately.
Download this from here.