You can install the SettingSanity extension from the Mozilla add-ons website. The installation of this add-on requires you to restart the web browser and it can be used immediately after the restart. After the restart, you will see no change in the main Firefox window. But if you open the Options window for Firefox (through Tools → Options menubar or from Firefox → Options → Options), you would see new options listed under the Content section.
Under the General section in the Firefox options, it also adds the options to disable tab bar when only one tab is being used. The option Always show the tab bar can be used to toggle the tab bar when there is no real need of it.
You can download and install the SettingSanity add-on from https://addons.mozilla.org/.