How to use it? Rated 4 out of 5 stars

I am probably missing something because I can't find screenshoted menu anywhere. What other addon I am supposed to have or where do I enable it?

Edit: Thanks for a quick reply. Now when I can find it it looks pretty good :-).

This review is for a previous version of the add-on (1.0.1). 

Well, you're right that it's not easily discoverable by itself, and it lacks an explanation in the addon description.
To enable it, just open the responsive design mode that you can find in the Web developer sub-menu.