Hiding the Firefox tab bar completely can be achieved through custom CSS, but it requires some manual steps since there's no direct option to disable it in Firefox's user interface. Here's how to do it:
- Type about:configin the Firefox address bar and press Enter.
- Accept any warnings and proceed.
- Search for the following setting: toolkit.legacyUserProfileCustomizations.stylesheets.
- Double-click it to set it to true.