A minimalist, simple, mouse centered userChrome — heavily inspired by Cascade.

Preview:

How to use a userChrome.css theme

1. Type about:config into your URL bar. Click on the I accept the risk button if you're shown a warning.
2. Seach for toolkit.legacyUserProfileCustomizations.stylesheets, layers.acceleration.force-enabled, gfx.webrender.all and svg.context-properties.content.enabled and set them to true.
3. Go to your profile folder:
   • Linux: $HOME/.mozilla/firefox/######.default-release/
   • MacOS: Users/[USERNAME]/Library/Application Support/Firefox/Profiles/######.default-release
   • Windows: C:\Users\[USERNAME]\AppData\Roaming\Mozilla\Firefox\Profiles\######.default-release
4. If it doesn't exist already create a folder called chrome.
5. Clone this repositoy into the chrome folder.