I often need to know the viewport or screen size when developing the style sheet for a site. It took a little bit to understand where to find the built in screen ruler in Firefox. Here are steps.

1. Hit f12 to bring up the developer tools.
2. Click on the gear to bring up the settings
   
3. Scroll down till you find “Toggle rulers for the page”.
   
4. Once that is checked you will see the ruler icon at the top of the toolbar.
   
   
5. Clicking on the ruler icon will display the ruler on the page.
   

I hope this helps someone else out there…