I only wanted to use display window size and change size but they aren't working in developer edition :(

A classic and very useful addon. For those using a FF browser with XUL support; the version 2.0.1 works fine in Waterfox (FF56) whereas v1.2.13 works in Basilisk (FF52)

Web Developer includes CSS tools, forms tools, HTML/CSS validation tools, & more. I recommend this extension for web developers.

good word. a very helpful tool

My go to developer tool in Firefox and Chrome. Reliable, easy to use, able to inspect element in Responsive design and see how changes will look. Almost perfect (if the inspector could show these changes across all screen sizes, this would give it 10 stars)