hello, please [https://www.mozilla.org/firefox/update/ update firefox] and [https://www.mozilla.org/en-US/plugincheck/ update your plugins] to the latest version as soon as possible - by continuing to use such outdated and vulnerable versions you will put the integrity of your system and your personal data at risk! https://www.mozilla.org/security/known-vulnerabilities/firefox.html you can use an addon like the following to customize the user interface in firefox: https://addons.mozilla.org/firefox/addon/hide-tab-bar-with-one-tab/