install_extension_in_firefox.js

import webdriver from 'selenium-webdriver';
import firefox from 'selenium-webdriver/firefox';

const XPI_PATH = ... // xpi file path

const firefoxOption = new firefox.Options();
firefoxOption.setPreference('xpinstall.signatures.required', false);
firefoxOption.setBinary(firefox.Channel.AURORA);  // Set Developer Edition

const driver = new webdriver.Builder()
    .forBrowser('firefox')
    .setFirefoxOptions(firefoxOption)
    .build();

(async function() {
  await driver.installAddon(XPI_PATH);
})();

// You can start work with driver now...