Some of my picks to add to all other comments

• Gesturefy: Mouse gesture addon. It can also run custom javascript. For example I have a script that makes copying text, links, buttons easier.
• Imagus mod: Hover zoom addon with up-to-date sieves that actually work. The sieve team is quite good.
• BetterViewer: Image viewer although some buttons don’t work on firefox.
• Fastforward: circumvents annoying link shorteners
• Distill: Monitor webpage or feed for changes

I also have many ***monkey and stylus scripts.