firefox-css-theme

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.

Getting Started

[!TIP] You need Node.js to get 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).

Launch Firefox with live CSS hot-reloading:

npx firefox-css-theme start

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

For Theme Users

Navigate to the theme directory and install the stylesheets:

npx firefox-css-theme install

The toolkit will look for and install userChrome.css and userContent.css in the present working directory into your Firefox profile.

If multiple Firefox profiles exist, an interactive selection list will prompt you to choose one.

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

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

{
	"mcpServers": {
		"firefox-css-theme": {
			"command": "npx",
			"args": ["-y", "firefox-css-theme", "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.

Commands

firefox-css-theme create

Scaffold a new Firefox CSS theme project.

Usage

npx 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

npx 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 from the current working directory into a Firefox profile.

Usage

npx 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

npx 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