This article applies to older versions of Firefox (Firefox version 56 and below).
Firefox no longer has a setting for hiding the tab strip when you only have one tab but there is an add-on you can download to change this.
- Go to the Hide Tab Bar With One Tab page on the Add-ons website.
- Click the button and it will begin to download.
- Click when prompted to start the installation.
- Finally click when it pops up. Your open tabs will be restored after the restart.
Now when you only have one tab, the tab strip will be automatically hidden.