Very good Rated 5 out of 5 stars
No doubt, this is a very cute, handy and useful extension.
There are just some issues i noticed, which need improvement.
The extension does not works when a tab contains only a picture.
The same also applies to a new tab (about:newtab).
Its button is always highlighted, not indicating its status like in
previous versions (v0.4.7 and earlier).
If MeasureIt is activated in some tab, going to an other tab, its button
needs to be clicked twice to activate it on that tab too.
Also, sometimes the button needs to be clicked twice to deactivate
MeasureIt, but i'm not exactly sure under what conditions this occurs.
However, it's related to toggling tabs.
The bottom-right corner handler of the measure frame (which remains
when you click outside of the frame to close it) has a weird behavior.
Most of the times appears in random places on the page (even outside
of the frame) and dragging it, the new frame is random too.
For this issue, maybe has a relation the "NoScript" extension.
(All above have been noticed on Firefox 21)
I hope this report to be helpful a bit to the developer, to make this
handy tool even better! :-)