Quick note

This article can be applied both to Firefox and any of its derivatives, such as LibreWolf or Waterfox. In fact, LibreWolf currently is my choice for a modern web browser, since it provides a cleaner experience than Firefox and also enables several privacy-enhancing features by default.

Background

The default interface for Firefox or LibreWolf that is shipped out-of-the-box features the tab bar, the back/forward and refresh buttons, and some more stuff. Whereas most elements can be added or removed from the toolbar using the Hamburger icon -> More tools -> Customize toolbar, the toolbar itself cannot be hidden in any way.

For a modern web browser with multiple tab support such as Firefox or Librefox, hiding the tab bar might not be a typical usecase. However, sometimes I like to use profiles to isolate given webapps that require user authentication and/or web storage, such as Element, so that I can still remove session data from web browsing sessions. For such single-page profiles, it is preferred to get some extra vertical space by hiding the tab bar.

Required changes

Fortunately, Firefox allows tweaking the so-called user chrome beyond what is allowed from the GUI.

1. Find the directory with the profile. Typically, Firefox stores profiles on $HOME/.mozilla/firefox/, whereas LibreWolf uses $HOME/.librewolf.
2. Profile directories typically are named after a sequence of somewhat random characters, plus the profile name itself e.g.: qicbr59r.default.
3. Inside the profile directory, there should be a directory called chrome. If not, create it.
4. Create a file called userChrome.css inside the chrome directory, with the following contents:

#TabsToolbar {visibility: collapse !important;}
#titlebar {
    visibility: collapse !important;
}
#titlebar-buttonbox {
    visibility: collapse !important;
}
#nav-bar {
    visibility: collapse !important;
}
#main-window[sizemode="maximized"] #nav-bar {
    visibility: collapse !important;
}

Then, the following changes must be applied to about:config:

toolkit.legacyUserProfileCustomizations.stylesheets = true
browser.tabs.tabmanager.enabled = false

Restart the browser and enjoy!