Still works in older versions Rated 5 out of 5 stars
@Grg68gr and all: The only thing that's changed in the latest version of MeasureIt is the forced status bar icon. In any version of Firefox you can add a MeasureIt button to any other Firefox toolbar just by right-clicking that toolbar, choosing customize, then dragging the ruler icon where ever you'd like it. In my case I've always kept a MeasureIt icon up in my navigation bar for easy access along side other tools.
Sorry for the trouble but this was a change recommended to me by some folks at Mozilla. I'll research if there's an easy way to restore this status bar icon by detecting the browser version but am not sure how/if that's possible yet.