When I used to use the del.icio.us sidebar, you could press Ctrl + B and it would appear. Does the diigo sidebar have a similar shortcut key to make it appear and disappear?
Shortcut Keys in Diigo Sidebar?
firefox;diigo;browser addons
The keyboard shortcut for opening the diigolet sidebar appears to be disabled by default, so you if you open about:config and search for extensions.diigotb.keys.sidebarset enabled to true;extensions.diigotb.keys.sidebar.enabled = truethen control-alt-s can be used to open and close the diigolet sidebarextensions.diigotb.keys.sidebar.keys = control+alt+s