New in Firefox 40.
Starting in Firefox 40, you can overlay horizontal and vertical rulers on a web page:
The units are in pixels.
Since Firefox 59, the dimensions of the viewport are displayed near the top-right corner of the viewport.
There are two ways to toggle rulers for a page:
- Use the Developer Toolbar's (shift+f2)
- Use a dedicated button on the toolbar. By default this button isn't displayed, but you can opt to display it using the "Available Toolbox Buttons" section in the Settings. Once displayed, the "Toggle rulers for the page" button appears at the top right of the Toolbox, in the same place as the Settings/Options button.
Behavior to keep in mind when using rulers:
- The rulers command must be reapplied in new tabs and each page refresh.
- The command isn't permanent.