I'm in javascript + mocha + node land.

I have tried setting Useragent and 'user-agent' as keys on capabilities:

var webdriver = require('selenium-webdriver');
var ua = 'Mozilla/5.0 (iPhone; CPU iPhone OS 5_0 like Mac OS X)';

var driver = new webdriver.Builder().
     ...
     withCapabilities({ 'browserName': 'firefox',
        userAgent: ua,
        'user-agent': ua,
    }).
    build();

There is this answer which says to use a firefox profile, but that's not exposed. There is no driver.FirefoxProfile nor one exposed globally nor webdriver.FirefoxProfile nor driver.profiles etc.

I have tried Googling and looking the source and the documentation but there is nothing on this.

You cannot do it with Firefox, but you can do it with Chrome. It's undocumented:

var chrome = require('selenium-webdriver/chrome');

var opts = new chrome.Options();
opts.addArguments(['user-agent="YOUR_USER_AGENT"']);

var driver = new webdriver.Builder().
    withCapabilities(opts.toCapabilities()).
    build();