Can't you? You used to be able to show the tab bar only when there was more than one tab; I guess that has disappeared?

You can do this: Put this code into your userChrome.css file:

#tabbrowser-tabs .tabbrowser-tab:only-of-type, #tabbrowser-tabs .tabbrowser-tab:only-of-type + #tabbrowser-arrowscrollbox-periphery { display: none !important; }

#tabbrowser-tabs, #tabbrowser-arrowscrollbox { min-height: 0 !important; }