New in Firefox 59.
Starting in Firefox 59, you can measure a specific area of a web page using the Measure a portion of the page tool.
This tool is hidden by default. To enable its button:
- Go to the DevTools Settings.
- From the Available Toolbox Buttons check the Measure a portion of the page checkbox.
You will now see the Measure a portion of the page button at the top right of the Toolbox, in the same place as the Settings/Options button.
When you want to use the tool, click this button. Now when you mouse over the viewport, you'll see the mouse has a crosshair cursor with its current coordinates displayed beside it.
When you hold the mouse button down and then drag, you'll start to draw a rectangle, with its x, y, and diagonal dimensions displayed. The units are in pixels.
When you stop holding the mouse down, the rectangle that was displayed onscreen when you released the button will stay there until you click again, allowing you time to take screenshots, note the information down, etc.