Pixel Ruler Pro by Rebecca

Pixel Ruler is an extension 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 displays width, height, and coordinates of the start and end points, all labeled with corresponding metrics in pixels.

When you move your mouse, the rectangle adjusts its size and updates the metrics. To activate the addon, press the toolbar button once. To disable the addon, press the toolbar button again. This addon works with mouse clicks on desktop machines and touch input on touch-enabled devices.