I have a toolbar from Gamers Unite! that I use for the "active friend" function on facebook games. This function builds a 'memory' of people who send you gifts in return, and picks them from the list of friends provided by the game, so non-active players are ignored.

This toolbar shows up in my "view toolbars" tab, shows up as "enabled" in my Firefox extensions manager, but there is a blank space where the toolbar should be in my browser header, and the "active friend" function does not show up when I go into a game's friend list.