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.

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

Exactly what I needed!

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.