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 2.0.3 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)

Na dann ist ja alles TUTTI

A must have for any web developer! I prefer this WebExtension (v2.0.1) than the old one (XUL extension).

Genial!!!

It's crap after 56.02 Firefox. Same shitty thing as on Chrome. Still have FF 56 and FF Nightly. I see no other workaround.

Unfortunately version 1.2.13 doesn't work with FF 62.x
Suggestion to developer: Go back and fix 1.2.13 to work with FF 62.x

if you are a developer, this is a must have extension.