In Firefox 23, as part of an effort to simplify the Firefox options set and facilitate future improvements to Firefox, the option to hide the tab bar was removed.
Fortunately, this can easily be resolved if you desire the keep tabs hidden. You can install "Hide tab bar with one tab", an extension hosted on Mozilla's add-ons site, which will restore the ability to hide the tab bar.
Thank you and I hope this helps!
Last Update: 2013-08-08