MeasureIt is a Firefox that enables you to measure the size of parts of the page you are viewing. MeasureIt shows up as a small button in the lower left corner of the Firefox window.
When you click on the button, the browser window will be overlaid with a whitish layer and the cursor turns into a cross. Click and drag the cursor over the area you want to measure.
A floating element will show the size of the select area. Clicking on the red square with the white cross will close the ruler but keep the whitish overlay. Pressing Esc on the keyboard removes the overlay.