Rated 5 out of 5 stars
To make this work with FF 3.6.* you need to create a chrome.manifest inside the .xpi instead of the deprecated .rdf files (see: https://developer.mozilla.org/en/Chrome_Registration#section_26). It worked for me.
I posted an example code for the chrome.manifest of this extension here: http://pastebin.com/f4191c637
I've been using this extension for ages and it's pretty useful, hopefully there'll be an official update someday.