fxcss.
THE FIREFOX THEME TOOLKIT

Your browser.
Your kind of Firefox.

fxcss is a command-line toolkit for Firefox themes. Preview a new look safely, edit your own theme live, and catch changes as Firefox evolves.

Find your starting point See the difference

Open source · macOS, Windows & Linux · Python + Firefox

What do you want to do?

From your first theme to your next release.

01

Try

Explore a new look in a temporary Firefox profile before installing it in your everyday browser.

Try a theme
02

Build

Create your own theme. Find the right selectors and see your CSS changes in Firefox as you save.

Build a theme
03

Maintain

Keep your theme working across Firefox releases with screenshot comparisons and compatibility checks.

Maintain a theme
CATCH A SMALL CHANGE BEFORE IT SHIPS.
Comparison example
Firefox 155.0.1 · macOS
Before · #4f6ef2After · #ff7139
Starter theme after the active tab accent changed to orange
Starter theme before the change, with a blue active tab accent
Drag the image to compare
Reveal before
50%
0.09% of the full capture changed
Actual starter-theme captures · fxcss 0.22.1 · 2026-10-06Full comparison Capture details

One CSS value changed the active tab. fxcss shows exactly where.

How comparison works
READY WHEN YOU ARE

A small install. A lot of possibility.

Run guided setup on macOS or Linux. Choose what you want to do, then get the tools and next steps.

curl -fsSL https://fxcss.com/install.sh | bash

Already have pipx? pipx install "fxcss[images]"

Read the setup script Installation guide