Highly Recommended!

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)

It's the only plugin that I found which displays a fluid grid and thus consents to replicate a bootstrap 4 grid.

It misses an useful feature though: the grid is not persistent after you refresh the page, you have to always recreate the grid, which is obviously a pain in the ass...

Some suggestions:
- add a toggle to show/hide the grid without deleting it after a page refresh
- add the possibility to invert the colors of the columns and the gutters (columns are transparent, gutters are colored)
- add the possibility to add different grids for different breakpoints
- add the possibility to save (with a name) different grids settings

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.