Firefox CSS Theme MCP Server

Firefox CSS Theme

A CLI toolkit for scaffolding, compiling, live-debugging, and installing Firefox UI and CSS themes (userChrome.css and userContent.css). It also provides a Model Context Protocol (MCP) server for AI-assisted inspection and styling.

Requirements

• Node.js >= 20
• Firefox Browser

Getting Started

Installation

Install as a development dependency within your theme repository:

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

Once installed, run it directly with npx firefox-css-theme [command] [option] or use firefox-css-theme in your npm scripts:

{
	"scripts": {
		"create": "firefox-css-theme create", // scaffold starter userChrome.css and userContent.css files
		"start": "firefox-css-theme start", // launch Firefox with live stylesheet bundling and hot-reloading
		"profiles": "firefox-css-theme profiles", // list all detected Firefox profiles
		"install:theme": "firefox-css-theme install" // install compiled stylesheets into a Firefox profile's chrome folder
	}
}

[!NOTE] userChrome.css and userContent.css must be located at the project root directory by default; otherwise, specify their paths using the -c, --chrome and -u, --content options.

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

Command Line Interface

firefox-css-theme create

Scaffold starter userChrome.css and userContent.css boilerplate files with Firefox namespace headers.

Usage

firefox-css-theme create [options]

Options

firefox-css-theme start

Launch Firefox with live stylesheet bundling and hot-reloading.

Usage

firefox-css-theme start [options]

Options

firefox-css-theme profiles

List all detected Firefox profiles and their directory paths.

Usage

firefox-css-theme profiles

firefox-css-theme install

Compile and install stylesheets directly into a Firefox profile's chrome/ directory.

Usage

firefox-css-theme install [options]

Options

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

firefox-css-theme mcp

Start the Model Context Protocol (MCP) server over standard I/O for AI-assisted inspection and styling.

Usage

firefox-css-theme mcp [options]

Options

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 capabilities enabled.
• close_browser: Terminates the browser instance.
• customize_toolbar: Activates or controls the Firefox "Customise Toolbar" mode.
• get_ui_tree: Dumps the hierarchical DOM tree of the chrome window.
• 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 stylesheets in the live UI (target: "chrome" | "content").
• remove_theme_css: Removes an injected stylesheet by id (target: "chrome" | "content").
• list_theme_css: Lists all currently injected custom stylesheets (target: "chrome" | "content").
• take_ui_screenshot: Captures a screenshot of the browser window or a specific UI component.
• execute_chrome_javascript: Runs privileged JavaScript in the browser chrome window context.