Firefox Oneliner

Screenshot

Minimal: Oneline Proton has been made trying to keep all the Proton experience, but with a cleaner interface.
Practical: The oneline interface allows you to keep a higher visible space and focus on what matters.
Clean: back/forward buttons keep hidden when disabled, and some url icons are hidden until you hover them.
Flexible: Support for compact, normal, and touch densities.
Themeable: You can still using custom themes with full compatibility.
Customizable: You are able to change settings like navbar's width, animation speed, and hidden elements at the top of the userChrome.css file.

In the searchbar, type about:config. A dialog will be shown to you. Press the I accept the risk button.
Search for toolkit.legacyUserProfileCustomizations.stylesheets and change it to True.
Go to your Firefox profile:
- In the searchbar, type about:support and press Enter.
- Search for Profile Directory and click on Open Directory button.
Create a folder and name it chrome (with lowercase).
Paste userChrome.css file into the folder.
Restart Firefox

You can change some settings inside the userChrome.css file.
Use --navbarWidth variable to change the width of the navbar (you can use px units for a fixed width, or vw units for a percentage one).
Use --animationSpeed variable to change (or disable) animations.
Hide or show the elements you want by commenting or uncommenting them.

This Firefox userChrome.css inspired by Oneline Proton.