What are the best Firefox extensions?

Question:
What are the best Firefox extensions?

Answer:
Maybe they're not extensions.

Source:
"Control animations with about:config" [1]

If you've never used about:config, you're missing out. Type about:config into your Firefox address bar and hit Enter. One easy configuration change is limiting animated images. Double-click image.animation_mode and change it to none to stop all animated images, or once to let them run through once. You can revert to the default behavior with normal."