not there Rated 1 out of 5 stars
installed it and its on my add-ons list and enabled however, its not showing up in the ad on bar. boooooThis review is for a previous version of the add-on (0.4.10.1-signed).
Thanks for the feedback! Though I wish it didn't merit a 1 star rating (ouch!) you can right-click on any toolbar in Firefox and choose "Customize". In there you'll find the MeasureIt ruler icon that you can drag to any toolbar you choose.
I hope to create an update that will auto place it in the add-ons toolbar soon.