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).
Only proceed if you are comfortable with advanced settings and understand the potential impacts.
- Type about:config in the address bar and press EnterReturn.
A warning page may appear. Click to go to the about:config page. - Find the browser.compactmode.show preference.
- Switch the toggle to true and then close the tab.
- Click the menu button to open the menu panel.
- Click .
- Choose .
- At the bottom of the panel, click .
- Choose
- Click .