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.
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.
Scrolls the page smoothly when scrolling the mouse wheel (or keyboard arrows if enabled).
The greatest user script engine on the Internet (a fork of Greasemonkey).
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"!
Changes the Location Bar of Firefox into a Breadcrumb display with rich and interactive features.
Automatically find user scripts on Userscripts.org (requires Greasemonkey or Scriptish)
This is a basic FireFox extension which simply scrolls the page down (and optionally up again) automatically at an adjustable rate.
Search within current site from the search bar, or from the context menu, or by drag-and-drop into the search bar.