Not working here either - that siad it DOES work sporadically on restarting FF and then the next time gone again. I would assume they are working on it. Hope so as its a great little tool.
A note from the developer "Thanks for the heads up about the conflict with Firefox 1.5.0.6 - I'm just finishing up the last day of my honeymoon (stuck in an airport in Ireland right now.) I should be able to get this fixed in the next day or two when I arrive back home, thank you for you patience!" https://addons.mozilla.org/firefox/500/
I think its been there a while now (just got back from holiday) but mine wasnt working so I did "tools" extensions" "updates and all is fine now sorry if this is "old" info' for most of you
What i've done is uninstalled the firefox frm my PC . then downloaded a fresh copy of the setup file and then reinstalled. Now it is working OK.