Opens a sidebar to manage containers
To open the sidebar use Ctrl/Cmd+Shift+Period.
Firefox lets users override 'chrome' level styles. Add a chrome directory and create a chrome/userChrome.css
file with the following contents:
@namespace url("http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul");
#TabsToolbar {
visibility: collapse;
}
When firefox is reloaded you won't have any horizontal tabs.