Found the icon again. Rated 5 out of 5 stars

I have the navigation toolbar hidden. The ruler icon found its way to the navigation toolbar. Moved it back to the Add-on bar (where it should be by default!) and it's usable and visible again.

This review is for a previous version of the add-on (0.4.13.1-signed). 

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 review is for a previous version of the add-on (0.4.13.1-signed). 

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.

~ Kevin

Downgraded to 4.7 Rated 4 out of 5 stars

I'll give that 5th star back (and donate!) when the latest, greatest again allows placement of the on/off toggle in the status bar. I found 4.8 to be comparably unusable.

This review is for a previous version of the add-on (0.4.8.1-signed).