I am thankful for the (somewhat) recent implementation of the sidebar and vertical tabs in Firefox, particularly the ability to have the sidebar completely hidden for minimal disturbance. I have found three states of the sidebar: ¹ Expanded (with tab title text), ² Minimized (only tab favicons), ³ Hidden (no sidebar visible).
Currently the [Hide/Show sidebar] button as well as the shortcut [Ctrl+Shift+Z] can only toggle between ¹ Expanded and either ² Minimized or ³ Hidden. To change between ² Minimized and ³ Hidden, the user must go through the 'customize sidebar' settings pane and toggle the 'Hide tabs and sidebar' checkbox.
I propose that it can be a preferable feature if the [Hide/Show sidebar] button and the shortcut can be configured such that the user can cycle between all three states instead of two of the three.