Prior to version 24, Firefox included by default an option to completely hide the tab bar if only one tab was open. Firefox 24 killed that feature. If you want to get it back, these two extensions will get the job done.
If all you want to accomplish is replacing that one aspect of Firefox's functionality, this addon is the way to go. It does one thing, and only one thing. In fact, it was developed specifically in response to Mozilla killing this feature in 24.
If you want a little more functionality, try Hide Tab Bar. It gives you the option to hide the tab bar completely, assign customizable keyboard shortcuts, add a delay, show the tab bar when you open a new tab, and, of course, hide it when there's only one tab open.