Firefox CSS Theme

A toolkit for Firefox CSS theme authors to scaffold, bundle, and live-debug theme repositories. It also provides end users with an automated installer.

A Model Context Protocol (MCP) server is also available for AI-assisted DOM inspection and live styling.

Requirements

• Node.js >= 20
• Firefox Browser

Getting Started

For Theme Authors

Scaffold a new theme project using npx:

npx firefox-css-theme create

This interactively prompts for your theme name and stylesheet targets (userChrome.css, userContent.css, or both), creating a minimal npm package with helper scripts and dependencies.

Alternatively, add firefox-css-theme as a development dependency to an existing project:

npm install --save-dev firefox-css-theme

Add helper scripts to your package.json:

{
	"scripts": {
		"start": "firefox-css-theme start", // launch Firefox with live CSS hot-reloading
		"install:theme": "firefox-css-theme install" // installer script for users
	}
}

[!NOTE] Theme entry files (userChrome.css and/or userContent.css) must be located at the repository root directory.

For End Users

When users clone your theme repository, they only need to run the installer:

# Clone the repository and install dependencies
npm install

# Installer the CSS theme
npm run install:theme

If multiple Firefox profiles exist, an interactive selection list will prompt you to choose one. To skip the selection, specify the target profile directly with -p, --profile <name>.

For the full list of CLI options, run npx firefox-css-theme --help in your project.

Commands

firefox-css-theme create

Scaffold a new Firefox CSS theme package.

Usage

firefox-css-theme create [name] [options]

Options

Option	Description
-t, --target <target>	Stylesheet target: both, chrome, or content
-f, --force	Proceed without warning

firefox-css-theme start

Launch Firefox with live stylesheet bundling and hot-reloading.

Usage

firefox-css-theme start [options]

Options

Option	Description
-b, --binary <path>	Path to Firefox executable
-p, --profile <name>	Firefox profile name
--no-watch	Disable file watching
--headless	Run in headless mode
--nova-ui	Enable Firefox Nova UI

firefox-css-theme install

Install compiled stylesheets into a Firefox profile.

Usage

firefox-css-theme install [options]

Options

Option	Description
-p, --profile <name>	Firefox profile name
-m, --merge	Merge with existing stylesheets
-f, --force	Proceed without warning

[!WARNING] This operation will overwrite existing CSS themes in the profile unless --merge is specified. A backup is advised.

firefox-css-theme mcp

Start the Model Context Protocol (MCP) server.

Usage

firefox-css-theme mcp [options]

Options

Option	Description
-b, --binary <path>	Path to Firefox executable
-p, --profile <name>	Firefox profile name
--headless	Run in headless mode
--nova-ui	Enable Firefox Nova UI

MCP Server Usage

The MCP server enables AI assistants to inspect the Firefox chrome DOM, query computed styles, inject custom CSS, and capture UI screenshots in real time.

Configuration

Add the server to your preferred client or development environment:

Claude Code

claude mcp add firefox-css-theme -- npx -y firefox-css-theme mcp --nova-ui

Codex

codex mcp add firefox-css-theme -- npx -y firefox-css-theme mcp --nova-ui

IDE MCP Configuration

Using npx:

{
	"mcpServers": {
		"firefox-css-theme": {
			"command": "npx",
			"args": ["-y", "firefox-css-theme", "mcp", "--nova-ui"]
		}
	}
}

Or using a local build:

{
	"mcpServers": {
		"firefox-css-theme": {
			"command": "node",
			"args": [
				"/path/to/firefox-css-theme/dist/cli.js",
				"mcp",
				"--nova-ui"
			]
		}
	}
}

Available MCP Tools

• launch_browser: Launches Firefox with chrome debugging.
• close_browser: Terminates the browser instance.
• customize_toolbar: Toggles the Firefox "Customise Toolbar" mode.
• get_ui_tree: Dumps the hierarchical DOM tree of selected UI element.
• query_ui_elements: Queries elements matching a CSS selector in the browser chrome.
• get_computed_styles: Extracts computed CSS property values of a specific UI element.
• inject_theme_css: Injects a stylesheet in the live UI.
• remove_theme_css: Removes an injected stylesheet.
• list_theme_css: Lists all currently injected stylesheets.
• take_ui_screenshot: Captures a screenshot of a specific UI component.
• execute_chrome_javascript: Runs JavaScript in the browser chrome window context.