I've been using this extension for several years now. This is probably the best extension for FireFox web browser that was ever created. Not only it allows you to go to the next tab using Alt - K (a truly innovative look at the old vim-style keybindings), but also go to the previous tab using Alt - J.
While some people may be opposed to using inverted keybindings as a default, I truly think that this **is** the way it was supposed to be. I even have my vim and wm setup with inverted vim keys.
All and all, this is truly a great extension. I hope to see more extensions like this.