Web developers know that finding exact measurements of anything that isn’t an image can be a pain. MeasureIt attempts to make life easier by putting a button in the bottom-left corner of Firefox’s window. Clicking on the button will put a semi-transparent white layer over the webpage (I don’t know why honestly), and lets you click and drag a portion of the screen to see its exact measurements. The addon isn’t perfect though, as you can’t select particular elements of a page like Colorzilla (an excellent color picker for Firefox) does.
All-in-all, MeasureIt gets the job done quick and pretty easily, so head over the the MeasureIt download page. At the time of this writing it works for any edition of Firefox from 1.0 to 3.0.