Please note that PrefBar is a legacy Addon which will not work in Firefox 57 and will not receive an update to do so. Please look at my other Addons which contain replacements for some of the buttons on PrefBar. Please also have a look at this list of replacements or plans for replacements of buttons: PrefBar to single Addons migration list
All items may be placed and grouped freely on PrefBar's bar. The bar itself may be moved using the browser built-in "toolbar customize" feature to anywhere you like (even in foreign toolbars, next to the menubar, ...).
A full list of built-in buttons is available here: http://prefbar.tuxfamily.org/help/buttons.html
You may create your own buttons (simple documentation included) or download more buttons from this page: http://prefbar.tuxfamily.org/buttons.html
For bug reports, please use the Bugzilla installation here: http://prefbar.tuxfamily.org/bugs.html