Great! and a key-navigation suggestion... Rated 4 out of 5 stars
Great addition! It should go in base Firefox ASAP -- because it provides keyboard jump to (almost) any tab without using the X0!x+! mouse.
I wish either the active number navigation extended past 10 into the double digits (because that's how many tabs I typically have open) -- or their were some vi-style keys mapped into the alt-A (Tabs) menu -- another way of extending keyboard-based navigation past the tenth tab.