Install the Sidebery add-on.
Right-click on the sidebar and select Compact mode -> Hide sidebar -> Enable compact mode.
Activate the mode configuration.

Tips:

Add user.js to your Profiles directory to enable Browser Toolbox Mode.

// user.js

user_pref("devtools.chrome.enabled", true);
user_pref("devtools.debugger.remote-enabled", true);

user_pref("xpinstall.signatures.required", false);
user_pref("config.trim_on_minimize", true);

Browser Toolbox Mode is located under Tools -> Browser Tools.
For quick style previews, utilize the Style Editor.
Customize navigation and sidebar colors via about:config -> zen.theme.gradient.show-custom-colors -> true.
Disable transparent backgrounds: about:config -> transparent -> false.
zen.theme.gradient