Toolbars and sidebars
For Firefox users who use/play StumbleUpon, RuneScape, Facebook, e.t.c. (I use those three, no more/less) toolbars are a "must". You can chose between one or several toolbars, if they're to be visible or not. All you have to do is go to Tools - Add-ons - Get Add-ons and search in the top left box, like with almost all Firefox extensions