I hope this helps someone someday, I needed to set user extension for a selenium test suite, dynamically in the code as i was starting the Selenium server via same. public void startSeleniumServer(String port) { rcc = new RemoteControlConfiguration(); rcc.setPort(Integer.parseInt(port)); try { seleniumServer = new SeleniumServer(false, rcc); seleniumServer.start(); } catch (Exception e) { throw new IllegalStateException("Can't start selenium server", e); } } public void stopSeleniumServer() { if (seleniumServer != null) { seleniumServer.