Add-ons I didn't install but that still have an interesting idea.
Add-ons I install when I'm at a public computer that I'm going to be using for a while. (Of course, the computer must first have Firefox and not restrict add-on installation).
The add-ons I always add to other people's computers after installing Firefox for them. Don't forget to install Flash, too, from http://get.adobe.com/flashplayer/.