A few more:
Bookmark Duplicate Detector: Detects Duplicate Bookmarks when bookmarks are added and specify where is the previous URL.* You can also search and delete duplicates URL already in your Bookmarks.
* Not sure about that first bit in Firefox 3.
Statusbar Clock: Display the date and time in your browser's status bar. Or just the date, or just the time, or maybe a cheerful message to brighten someone's day. It's quite customizable.
Firefox: 2.0 – 2.0.0.* [works fine in Firefox 3 if you overide version checking]
TagSifter: Firefox 3 lets you tag your bookmarks, but it doesn't give you a great way to browse your bookmarks by their tags. TagSifter tries to.
Firefox: 3.0 – 3.0.*
Update Bookmark: Allows bookmark properties to be set to the current page's properties or to clipboard text quickly via the context menu.
Firefox: 3.0b1 – 3.1a1pre