Starting with Firefox version 89, the compact mode density option was removed from the Customize panel but you can still activate it in the Configuration Editor (about:config page). Warning: Changing advanced preferences can affect Firefox’s stability and security. This is recommended for advanced users only.

1. Type about:config in the address bar and press Return.
   A warning page may appear. Click Accept the Risk and Continue to go to the about:config page.
2. Find the browser.compactmode.show preference.
3. Switch the toggle to true and then close the tab.
4. Click the menu button to open the menu panel.
5. Click More tools.
6. Choose Customize toolbar….
7. At the bottom of the panel, click Density.
8. Choose Compact (not supported) from the menu options.
9. Click Done.