Page Grid Ruler is a Firefox extension designed for users needing precise layout measurements on web pages. It provides a customizable grid view and a dynamic ruler that displays width, height, start, and endpoint measurements in pixel units. This tool is particularly useful for web designers and developers who require accurate alignment and spacing while working on their projects.

To utilize the extension, users simply navigate to a desired page and activate the action button to enter grid mode. The grid can be tailored according to preferences, and adjustments to the grid size and thickness can be made via right-click menus. Although the extension does not support measurements in millimeters or inches, it effectively serves its purpose by offering pixel-based measurements for straightforward layout design.