an excellent extension, currently missing from AMO Rated 5 out of 5 stars
Stop. Point at the install button. I'm getting hide_my_ass_ip_checker-1.5.1-fx.xpi here. That seems to be spam for hidemyass-dot-com.
I've been using this extension and its predecessors since the Seamonkey days, before Firefox. It's still at Piro's site and at GitHub, but it's missing from this page.