Online Page Ruler

Page Ruler is an add-on that allows you to measure distances on a webpage.

When you move your cursor over the website, it draws a rectangle ruler. The width, height, start, and finish of the ruler are all labelled with the respective metrics in pixels. When you move your mouse, the rectangle expands and all the numbers update. To enable the addon, tap the toolbar button once. Similarly, to deactivate the addon, hit the toolbar button once again. On desktop computers, this addon works with mouse clicks.