New in Firefox 40.
Starting in Firefox 40, you can overlay horizontal and vertical rulers on a web page:
The units are in pixels.
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.
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.