Download Firefox 55 beta and set
capabilities.setCapability("acceptInsecureCerts", true);
Here is my code that works for Firefox 55 beta:
DesiredCapabilities capabilities = new DesiredCapabilities();
capabilities.setBrowserName("firefox");
capabilities.setCapability("acceptInsecureCerts", true);
RemoteWebDriver driver = new RemoteWebDriver(Environment.remoteWebDriverURL, capabilities);