OS: Windows, macOS, Linux
Selenium Version: 3.5.0
Browser: Firefox
Browser Version: 56

Once Firefox supports headless mode on all three platforms (starting with Firefox 56, currently in beta and due to be released next month), the selenium-webdriver package should support configuring Firefox to run headlessly the same way it supports Chrome running headlessly, via a headless() method on the Firefox driver's Options class, so that you can drive Firefox headlessly via:

const { Builder } = require('selenium-webdriver');
const firefox = require('selenium-webdriver/firefox');
const driver = new Builder()
.forBrowser('firefox')
.setFirefoxOptions(new firefox.Options().headless())
.build();

(It would additionally be useful to be able to set the headless window's size via the equivalent of Chrome's windowSize({width, height}) method.)