ʉۢ I like Add Bookmark Here, which adds an similarly titled menu item to every submenu in the Bookmarks menu enabling easy bookmarking in any folder.

 â€¢ Dictionary Tooltip is one of my favorites. It displays the definition of a selected word in a kind of “scrollable tooltip” right on the page (as opposed to similar extensions that open a new window or tab, like Answers or DictionarySearch). There are several ways to invoke its inline window (it’s Control-Shift-D by default), but a double-click with the Control key down works for me.

 â€¢ Then there’s Uppity, which adds an “Up” toolbar button that can be added to the navigation toolbar (and optionally the status bar, too), and makes navigating up a website’s directory structure as easy as clicking the button or pressing Alt-Up (Option-Up on Macs).

ʉۢ Mycroft can add any of dozens of new search engines to the Search bar (like OneLook, Wikipedia and the IMDb), and SearchPluginHacks makes it easy to remove them.

 â€¢ Finally, Tabs Menu adds, appropriately enough, a Tabs menu to Firefox’s main menu bar. This simple feature makes navigating the current window’s open tabs much, much easier.