So I updated to FF23 and was unhappy that it now forces you to lose the real estate from displaying a single tab instead of just showing tabs when one has multiple open. I guess this was the default before, but we were able to shut it off in preferences. Apparently this was done to prepare folks for an up-coming redesign that will hold on to that space for some reason -- this doesn't leave me feeling very optimistic.
Anyway, here's a simple work-around: https://addons.mozilla.org/en-US/firefox/addon/hide-tab-bar-with-one-tab/