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.
- Hit f12 to bring up the developer tools.
- Click on the gear to bring up the settings
- Scroll down till you find “Toggle rulers for the page”.
- Once that is checked you will see the ruler icon at the top of the toolbar.
- Clicking on the ruler icon will display the ruler on the page.
I hope this helps someone else out there…