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.
Status bar icon Rated 5 out of 5 stars
The latest version removed the status icon since there's now an add-ons toolbar in Fx 4. However, anyone still using Fx 3.6 or below can get that status icon back by using version 0.4.7 of MeasureIt. You can get that version at https://addons.mozilla.org/en-us/firefox/addon/measureit/versions/
Cheers and thanks everyone for using MeasureIt!