It's indeed the only add-on that is doing what I would like to have.
A suggestion is to look at the similar, but more advanced, version in Chrome: 'Design Grid Overlay' (https://chrome.google.com/webstore/detail/design-grid-overlay/kmaadknbpdklpcommafmcboghdlopmbi)

Great and helpful extension.

While diving deeper into this extension, I became aware of something missing: the ability to edit screen sizes.
The user is indeed allowed to change the container size but not the screen sizes, like in this example: https://yadi.sk/i/Jgo43n2i3NJsQe
This makes working with custom grids a little bit harder.