This provides the undocked (floating) Sidebar. You can place it to your favorites position. (The undocked Sidebar can collapse/expand itself automatically with mouseover.)
Some features on versions older than 3.2 are removed on the version 4. I recommend you to use All-in-One Sidebar ( https://addons.mozilla.org/firefox/addon/all-in-one-sidebar/ ) if you want such features: one keyboard shortcut to toggle the sidebar, useful toolbar button, and so on.