The overlay (top right on viewport size change) shows and fades too quickly. Also it only works on the first tab when you have multiple tabs open and also the little overlay box prevents clicking on any content beneath it (even when not showing) it needs "pointer-events: none;" on the box .

It could be useful if these things were tidied up.

This addon has 2 major problems. The first is the width does not match the CSS viewport width or javascript window.innerwidth because it doesn't take into account the width of the scrollbar. The second is like others have said it disappears too quickly once you finish resizing the browser. It would be better if it remained for 5 seconds or so after resizing finishes.

Hopefully these issues will be fixed in later updates, but for now I'll keep using this bookmarklet.
https://seesparkbox.com/foundry/media_query_bookmarklet
And also this one which is another bookmarklet and does everything I wanted this addon to do.
https://codepen.io/jakob-e/pres/qEZZox

The width this extension reports while resizing is not the same number that media queries are seeing. For instance, when a media query determines the width is 510px for a certain CSS rule, this extension shows 492px, not 510.

I assume this has something to do with the scrollbar, but this should be taken into account so that it matches media queries.