The addon bar is not just meant for addons. Any Firefox button can be added to it! Recent versions of Firefox hide almost all the toolbars to present a minimal UI. If you miss some of the buttons you used frequently, it might be a good idea to add them to the addon bar. This turns the addon bar into your own custom button bar.
Another advantage is that the addon bar is thinner than any of the toolbars at the top. Thus, you save a bit of precious vertical space. The addon bar can be toggled easily using its keyboard shortcut (
Ctrl+/), which actually makes it much more convenient than the toolbars at the top.
Toggle the addon bar to make it visible if it isn't already so. Right-click any empty space in the tab bar at the top and choose Customize. Once the Customize dialog shows up, drag any button down to the addon bar. The button you drag can be in the Customize dialog or it can even be in any of the toolbars behind it. Cancel the Customize dialog and you are done!
Tried with: Firefox 12.0