Firefox Proton Square

Recreates the feel of Quantum with its squared tabs and menus. No rounded corners to be seen.

Disclaimer: Tested for Stable and Beta channels (Firefox 91+) on Linux

If you are like me and hate rounded tabs, this is for you.

These tweaks attempt to recreate the feel of Quantum with its squared tabs and menus, but in the Proton UI. As of FF91, the about:config option to disable proton ("browser.proton.enabled = false") has been deprecated, leaving CSS as the main way to revert the UI.

Features

Tab Design

• Square corners
• Connect with toolbar (no floating tabs)
• No spacer on left tab bar when windowed
• Colored tab context line above selected tab
• Divide Line between tabs

Menu Design

• Square corners on most popup menus
• Square toggle buttons

How to Install?

1. Enable userChrome.css Support.
   • Go to about:config
   • toolkit.legacyUserProfileCustomizations.stylesheets to true
   • svg.context-properties.content.enabled to true
2. Find Profile Directory.
   • Go to about:support
   • Open Profile Directory
3. Copy Files
   • create chrome directory at profile
   • download the userChrome.css and userContent.css files from this repository
   • copy the files into the chrome directory
4. Restart Firefox

Thunderbird is also supported.

Suggested Tweaks

about:config

• set browser.tabs.tabMinWidth to 150 px or desired width
• set widget.non-native-theme.gtk.scrollbar.round-thumb to false to square the scrollbar (linux only)
• change layout.css.devPixelsPerPx to scale the whole browser (1.0 represents 100% scaling)

Addons

• Stylus or similar for editing webpage CSS
• create custom rules to apply *{border-radius: 0 !important} to square most elements on webpages
• may result in webpage breakages, apply at own risk

Appendix

More examples and screenshots.