Let's suppose that you want to hide tabs when you are entering Settings. Just add navigation in your constructor:

function SettingsStackScreen({ navigation }) {
    navigation.setOptions({ tabBarVisible: false })
    return (
        <SettingsStack.Navigator>
            <SettingsStack.Screen name="Settings" component={SettingsScreen} />
        </SettingsStack.Navigator>
    )
}

This code should work.

1. Open your firefox "profile directory"

• Official docs how to do that: https://support.mozilla.org/en-US/kb/profiles-where-firefox-stores-user-data#w_how-do-i-find-my-profile
• Or just open:
  • Linux: ~/.mozilla/firefox/<profile>
  • OSX:
    • ~/Library/Application Support/Firefox/Profiles/<profile folder>
    • or ~/Library/Mozilla/Firefox/Profiles/<profile folder>
  • Windows: %appdata%\Mozilla\Firefox\Profiles\<profile folder>

1. Create directory chrome/ if it doesn't exist

2. Create file chrome/userChrome.css inside if it doesn't exist.

3. Add this text to the file:

   #TabsToolbar { visibility: collapse !important; }
   

4. Ensure the config toolkit.legacyUserProfileCustomizations.stylesheets is set to true (required for Firefox69+, the stable version since September 2019), see this tutorial.

5. Save the file and reload firefox. You should see no tab bar anymore.

P.S. Solution partly taken from here: https://www.ghacks.net/2017/09/27/tree-style-tab-is-a-webextension-now/

UPDATE 2022-10-24: Starting from Firefox-106.0, there is also a Tab Manager / Tab View that needs to be disabled. To disablet it, set browser.tabs.tabmanager.enabled to false