Page Ruler от Leonardo

Page Ruler is an add-on that lets you measure distances (in pixels) on a webpage.

It draws a rectangular ruler when you move your mouse on the page. The ruler has width, height, start and end all labeled with corresponding metrics in pixels. When you move your mouse, the rectangle changes its size and all the numerics are updated. To activate the addon please press the toolbar button once. Likewise, to disable the addon please press the toolbar button again. This addon works with mouse clicks in desktop machines. For touch-enabled devices, you can work with touch to draw the rectangle within the page.

If you have a feature request in mind or found a bug to report, please fill the bug report form on the addon's homepage.