Doesn't work as intented with new HTML features likes dialog or popover. Also doesn't work with stopped debugger which kinda defeats the purpose for a webdeveloper.

Why I can not move the rectangle that I have created?

Wow! Excellent! Shows a grid on which you select a rectangular zone to get it's pixel size.

This extension is great, but I'd request Leonardo to allow customization of the font color that shows up in Ruler mode because in some websites, there are black backgrounds which make the font indistinguishable from the background. Except for this, everything works great!