The greatest user script engine on the Internet (a fork of Greasemonkey).
Control what gets sent as the HTTP Referer on a per-site basis.
DOM Inspector is a tool that can be used to inspect and edit the live DOM of any web document or XUL application. The DOM can be navigated using a two-paned window displaying a variety of different views on the document and all nodes within.
Add more items to Add-ons Manager context menu.
For extension developers: allows automatically adding or updating browser extensions, e.g. with command line tools.
Gives a possibility to create custom toolbarbuttons...
Pretty-prints JSON content in the browser for easy, unobtrusive viewing.
This one is a Star Trek theme, using the LCARS interface known of Federation Star Ships (heavily used in The Next Generation and Voyager series). It's got lot of rounded borders and hover (mouse-over) effects, be aware that it may look and feel...
Add "Go Parent Folder" menu to context menu in The Library list view and Search result in Bookmarks Sidebar.