It is beyond me why the folks at Mozilla would do this, but in the latest release of Firefox (version 23), the ability to hide the tab bar when only one tab is open was removed. For some users (like myself) this is valuable vertical screen real estate that should not be wasted when not needed. Luckily, the simple add-on Hide Tab Bar With One Tab was created — it takes a split-second longer than it used to in native Firefox, but it's a lot better than nothing.

Install and enjoy! https://addons.mozilla.org/en-us/firefox/…