Rated 5 out of 5 stars
MeasureIt *works* with FF, incl. 3.5.6 (and latest as of today, 3.5.7) w/o any problems.
Please keep in mind that if you use NoScript it will NOT work for the page NoScript blocked JS on. You have to enable JS (even temporary) for that page to make MeasureIt work. Would be handy if MeasureIt could check that by itself and warn user if this is the case.