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It's a ruler Rated 3 out of 5 stars
Ruler gets laid on top of your view port, but you can't move the ruler to elements on the page. So there's no way to truly align objects on your page to the pixel.
Need to be able to move it similar to the way photoshop works. Maybe in the next version?
No documentation - can't get to work Rated 2 out of 5 stars
I got message that this was installed. But I see no button. I see nothing in the context menu. In fact, I don't see how to turn on / off this ruler anywhere and there is no documentation. So I'm going to have to rate this as not useful until someone replies with instructions on how to start it.
Issue is actually with Firefox, your Add-on Bar is hidden
If you hit CTRL+/ or CMD+/ or navigate to View > Toolbars > Add-on Bar, you will see your extensions in the lower-right of the window. Good luck.
Great Promise Rated 4 out of 5 stars
This addon overlays a ruler on top of your viewport. It makes it somewhat easy to analyze a site as it crosses multiple breakpoints. The ruler values are a little hard to read, but it does the job. I was hoping the ruler would surround the viewport instead of overlaying on top of it so I can see the entire site (including the portion underneath the ruler).
This app has great promise and I look forward to enhancements.