Defragments your Firefox "Places" database (history/bookmarks)
This greatly reduces the lag while typing in the address bar and the start-up time.
This extension features configurable automatic cleaning, periodic reminder, and internationalization.
Useful tools for the nightly tester.
For some reason, Mozilla doesn't have a Canadian localization team, but I know that even the British dictionary doesn't quite cut it for the truly patriotic.
More Advanced (hidden) configures
NoRedirect lets the user take control of HTTP redirects. It can be used to interdict an ISP's DNS search redirection hijacks, preview/screen "shortened" URLs (e.g., TinyURL), stop the annoying redirection of "smart" error pages, etc.
This extension provides advanced configuration for your browser. It allows you to easily change many additional options, which are not displayed in the Firefox Options dialog.
Display all Google services as buttons or as a space-saving dropdown menu next to your address bar. Reach services like Gmail, Google reader, Google maps, Google calendar, and many more in a single click from your browser.
Add useful extra features to Gmail, like hierarchical labels, an unread message icon on your browser tab, file attachment icons, and more.
This extension allows to upload/download files from your favorite websites using a simple interface. Below is the list of all websites supported by Fireuploader.
You can capture your screen and record your voice to make screencasts (tutorials, e-lessons, etc.).
This extension adds a new "FaviconizeTab" option to the context menu of the tab...
Customize Your Web (CYW) let's you customize your favorite websites with just a few clicks. What Greasemonkey is for developers is CYW for non-developers. You can remove/rearrange /modify elements, add keyboard shortcuts and do many other things.
For those people who used to use Internet Explorer and the option Show Picture. This extension, also called Show Picture, loads and displays the selected image without reloading the page. There are also menu options to Show All Pictures, Show...