Mozilla and Mozilla Firefox allow users to extend or completely alter existing functionality using extensions (typically small packages/applications). Rather than requiring someone to edit the source, these changes can be loaded like a plug-in, easily disabled or uninstalled. Extensions can be as simple as adding a single menu item or as complicated as completely changing the application interface.

Combo Tab Icons v0.1.4.4 (Firefox 2.0-3.0.*; screenshot):
Combines the site and close tab icons. The site icon is only visible when the tab is busy, displaying the throbber.
Delete Icons v0.1.2 (Firefox 0.9-2.0.0.*):
Delete site icons via the Tools or bookmarks context menu. (see bug 257791)
 
Note: No plans to update for 3.0 at this time.
Focus Last Selected Tab v0.9.5 (Firefox 2.0-3.0.*):
Focus Last Selected Tab v0.9.2 - old stable release (Firefox 1.5-3.0.*):
Focus the last selected tab when closing.
 
Similar to Gorgias' extension (of the same name), but contains only the core functionality and uses a more simplistic and reliable method to determine the last selected tab.
Open Tab Right v0.3.3.2 (Firefox 1.5-3.0.*):
Open new tabs to the right.
 
Note: Version 0.3+ no longer modifies addTab(), which should cause fewer problems with session-related extensions.
Plugin Manager v0.1.4.2.2 (Firefox 0.9-3.0.*; for Unix-like systems; screenshot):
Manage plug-ins via the Tools menu, no restart required (see also bug 19118).
 
Note: Deprecated by bug 339056 for Firefox 3.0.
 
Background / How it works:
  Firefox can use multiple plug-in paths, which commonly include the installation (/usr/local/firefox/plugins) and a per-user (~/.mozilla/plugins) directories. Since there is currently no interface for (un)loading plug-ins, the next best thing is to simply move the plug-in out of it's respective directory (into a disabled subdirectory) and ask Firefox to rescan for changes.
 
Caveats:
   Multi-user systems.
   File ownership and permissions.
 
Common issues:
   A greyed out plug-in means the plug-in's parent directory (or it's disabled directory) is not writable by the current user.
 
Bugs:
   Windows-unfriendly: plug-ins in the registry are not managed at this time (plugin.scan.plid.all, plugin.scan.*).
Permission Manager v0.1.1.2 (Firefox 0.9-3.0.*; screenshot):
Adjusts the content policy settings (allowing image, cookie, script, and related objects) via the Tools menu.
Popup Blocker Settings v0.2.1.2 (Firefox 0.9-3.0.*; screenshot):
Quickly toggle between three different modes of popup-blocking - Open all, Block unrequested, and Block all.
 
Adjusts dom.disable_open_during_load, dom.popup_allowed_events, and privacy.popups.disable_from_plugins.
Restore Scroll Position v0.5.9.10 (Firefox 2.0-3.0.*):
Restore Scroll Position v0.4 - old stable release (Firefox 0.9-2.0.0.*):
Restore scroll position for pages with no-cache and related headers. (see bug 215405)
 
News:
  * 0.5+ can restore text inputs if restorescrollposition.formData is true (default: false).
  * 0.5.9+ uses a history event listener for better integration and more reliable operation (mouse nav should work).
  * 0.5.9.9 has gone back to using the 'load' event and will no longer process page loads from Gecko's bfcache.
  * 0.5.9.10 removes the .reload pref (unused since 0.5.9.3), but will continue to restore on reload.

Related: developer.mozilla.org, addons.mozilla.org, mozdev.org, extensionsmirror.nl.

Feedback is welcome - mozilla+ext ! at ! gozer ! dot ! org (replace {char}{word}{char} with @ or .).