When using a vertical tab extension like Vertical Tabs Reloaded, you can hide the tab bar by adding the following line to userChrome.css:

TabsToolbar { visibility: collapse !important; }

See multi row tabs for instructions on how to find the userChrome.css file.

This mode may require changing the way new tabs open. Set browser.tabs.insertAfterCurrent and browser.tabs.insertRelatedAfterCurrent (in about:config) according to this table:

	insertAfterCurrent	insertRelatedAfterCurrent
open all tabs next to the current tab	true	whatever
open all tabs at the end of the tab bar	false	false
related tabs next to current, others at the end	false	true