It works. Was just what I was looking for. Tks Kameron! Only thing is it seems that the Firefox Rulers pixel width measurement is slightly different to Viewport pixel width.

Doesn't work - can't see the current size of my window anywhere, nor do a I see any way to add it to my toolbar etc.

No option to change the position of the little popup with the viewport size...
This is a problem when you are developing with WordPress..

Thanks for your work! Very helpful addon. But could you please increase a z-index? In WordPress, admin panel has 99999 z-index, and your ruler is not showing there. Thanks!

Does what it says. I prefer it to the default rulers because it doesn't cover as much of the viewport

Not working in latest dev edition! And btw: why only show it on resize and not just always???

Doesn't do anything, not showing size or anything at all. This garbage should be removed from Add-ons

WHERE does show this Add-on the current view port size?

Exactly what I needed!

I use this and think it works well. I find I must disable it when I want to interact with content on certain pages. Even though the add-on is not active (I'm not resizing the window), it seems the overlay element gets loaded and I either have to manually display:none the invisible element or disable to add-on.

You have to be very quick to catch the displayed viewport size before it disappears.
Is there any way to lengthen the time that the size display is visible, or maybe have a special key sequence to reactivate and redisplay the viewport size without having to resize the browser again?

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

Agree with previous comment, the numbers disappear way too fast. But, other than that, works great.

To the fellas saying it doesn't work: go to google.com. Try resizing your browser window, and look in the upper right corner of your viewport. Are you sure nothing happens?

For me, the only site it doesn't work on is this one (addons.mozilla.org). Works fine everywhere else.

This addon is a real joke, nothing happen.

It works, but the size fades far to quickly to be useful; keep it there for a few seconds.

Nothing happens after install (FF57 on Mac), so it doesn't seem to work. A pity, because an add-on like this would be really helpful.

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.

When installing it, doesn't seems to do anything..