They are referring to hiding individual tabs, not hiding the horizontal tab bar.
The legacy extension they are referring to is Tab Groups [0]. It allowed you to separate the tabs of a single session into groups. Selecting the active group would hide the other tabs.
The legacy extension they are referring to is Tab Groups [0]. It allowed you to separate the tabs of a single session into groups. Selecting the active group would hide the other tabs.
[0] https://addons.mozilla.org/en-US/firefox/addon/tab-groups-pa...