Firefox now has native vertical tabs as well as a multitude of add-ons that make use of the sidebar to manage tabs (for example "Tab Stash"). In order to reduce redundant information on screen, specially when using the sidebar add-ons, it would be nice to have a button that toggles the horizontal tabs. This would work as the dual to the "Turn on Vertical Tabs" option that we have in the context menu of the Tool Bar.