Maybe if it worked at all in any way whatsoever... Was looking forward to checking it out... Rated 1 out of 5 stars

Well, it is completely and totally inoperable I'm sad to say. There is no button. Also per the developer's comments on J Howard's review, I looked in the "Tools" menu and alas, there is no such menu item as "add on bar" since FF removed that long ago... Of course, due to this CTRL+/ is pointless as well. Perhaps you could take the time to move where the button is located as that is a pretty trivial thing to do.

Also, and just a thought here, given LarryBud's comment and to a large degree I really do see his point and honestly had I seen it prior to installing, I probably wouldn't have; however, if you were to make a pair of very feint lines, one from each ruler, that intersected at the cursor and had the position in px floating in a tooltip next to that. Well, THAT would be awesome!

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.

This review is for a previous version of the add-on (1.0.rev45.1-signed).