If you can't see the vertical tab sidebar you need to reopen it! Use the hotkey (Ctrl+Shift+V), the VTR icon somewhere in the right top corner (via default) or Firefox's default sidebar icon and switch in the sidebar header's dropdown menu to Vertical Tabs Reloaded. The default horizontal tab browser can't be hidden right now, that's a limitation in Firefox itself: https://bugzilla.mozilla.org/show_bug.cgi?id=1332447 Features:

arranges tabs vertical
ships 4 different styles for the tab sidebar
hides tab sidebar in fullscreen mode
hide/display manually the tab sidebar with a hotkey (Ctrl+Shift+V) or by clicking on the VTR icon
display the tab toolbar at top (default), bottom or hide it completely
show the tab status (unloaded, unready, busy) visually on the tabs

Feedback/Errors If you have found any errors or want to suggest improvements please feel encouraged to do so on GitHub: https://github.com/Croydon/vertical-tabs-reloaded/issues If you just want to say thank you, leave a comment on https://addons.mozilla.org/firefox/addon/vertical-tabs-reloaded/