Add-ons I didn't install but that still have an interesting idea.
Integrates location bar and search bar into one. Shows search and url suggestions. Quickly switch search engine using search keywords. Provides different auto-complete popup styles for compact displays.
The greatest user script engine on the Internet (a fork of Greasemonkey).
Scrolls the page smoothly when scrolling the mouse wheel (or keyboard arrows if enabled).
Anything between simple tweaks such as placing the findbar in the top corner, to the powerful "Find in All Tabs" tool, FindBar Tweak will make your find toolbar "just right"!
Never lose anything you type into a web form again! Lazarus securely auto-saves all forms as you type, so after a crash, server timeout, or whatever, you can go back to the form, right click, "recover form", and breathe a sigh of relief.
Automatically find user scripts on Userscripts.org (requires Greasemonkey or Scriptish)
Move Firefox Button to anywhere you want, and make it also available in Full Screen mode, or when Menu Bar is showing.
Changes the Location Bar of Firefox into a Breadcrumb display with rich and interactive features.
This is a basic FireFox extension which simply scrolls the page down (and optionally up again) automatically at an adjustable rate.