Fix for no "Add Overlay" button in newer versions of Firebug Rated 4 out of 5 stars
I've found a fix posted posted here:
Seems to have worked fine for me. Good luck! Hopefully some day the developer will update the plugin. I saw new versions of the plugin available on GitHub, but these versions had an entirely different (and much worse, in my opinion) interface and didn't seem to function at all.