Firefox Extension: Recently Used Tabs Indicator

I made a Firefox extension to make my life a tiny bit easier. I have dabbled in this before, but I have never published one. I have a problem with tabs. I often end up with too many open, and often several from the same site. So when I switch to a different tab to check something, and I want to go back, I am stuck scanning through a sea of tiny favicons trying to remember which one I just left, as…