A sine qua non!
It's one of my favourites, for the following reasons:
- It's useful, simple (does one thing and does it well!) and unobtrusive -- beautiful!
- Adds some Konqueror (KDE's default web browser) behaviour to Firefox: transplants a good idea of one piece of (Open Source) software to another
- It can be controlled using the keyboard, as well as the mouse
- It seems to be compatible with all future versions of Firefox (!)
- It has a nice icon -- as well as an inspired name!
It would be nice to have it in the context menu as well...