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.
Sorry I don't have one but I'm guessing it wouldn't be very simple as there are so many lemmy websites.