Yep. Button's gone missing :( Rated 2 out of 5 stars
This was a five star helper, but now it's pretty much unusable because of ff updates.This user has other reviews of this add-on.
I'm sorry you're having trouble with MeasureIt. I was just able to install the extension in Firefox 25 and the icon showed for me. What version are you using? Have you tried right-clicking on one of the toolbars in Firefox then choosing "Customize"? If you do that you should find the ruler icon in that popup and you can drag it into any toolbar you like.
Thanks for the feedback and I hope I can help turn that 2 star review around.