Firefox MCP Server

Instructions

Implementation Reference

• index-multi-debug.js:1070-1083 (handler)

  The core handler function for the 'execute_script' tool. It retrieves the target page, ensures the browser is running, evaluates the provided JavaScript script using Playwright's page.evaluate(), and returns the execution result.

  async executeScript(args) {
    this.ensureBrowserRunning();
    const { script, tabId } = args;
    const page = this.getPage(tabId);
    
    const result = await page.evaluate(script);
    
    return {
      content: [{
        type: 'text',
        text: `Script executed in tab '${tabId || this.activeTabId}'. Result: ${JSON.stringify(result)}`
      }]
    };
  }

• index-multi-debug.js:164-175 (registration)

  Registers the 'execute_script' tool in the ListTools response, including its name, description, and input schema specifying required 'script' parameter and optional 'tabId'.

  {
    name: 'execute_script',
    description: 'Execute JavaScript in the browser',
    inputSchema: {
      type: 'object',
      properties: {
        script: { type: 'string' },
        tabId: { type: 'string' }
      },
      required: ['script']
    }
  },

• index-multi-debug.js:167-173 (schema)

  Input schema for the 'execute_script' tool, defining properties 'script' (required string) and 'tabId' (optional string).

  inputSchema: {
    type: 'object',
    properties: {
      script: { type: 'string' },
      tabId: { type: 'string' }
    },
    required: ['script']

• index-multi-debug.js:1021-1033 (helper)

  Helper method used by executeScript to retrieve the Playwright Page object for the specified or active tabId.

  getPage(tabId) {
    if (tabId) {
      if (!this.pages.has(tabId)) {
        throw new Error(`Tab '${tabId}' not found`);
      }
      return this.pages.get(tabId);
    } else {
      if (!this.activeTabId || !this.pages.has(this.activeTabId)) {
        throw new Error('No active tab. Use create_tab or set_active_tab first.');
      }
      return this.pages.get(this.activeTabId);
    }
  }

• index-multi-debug.js:1400-1404 (helper)

  Helper method called by executeScript to ensure the Firefox browser instance is launched before executing scripts.

  ensureBrowserRunning() {
    if (!this.browser) {
      throw new Error('Firefox browser is not running. Please launch it first using the launch_firefox_multi tool.');
    }
  }