I'm launching wdio programmatically supplying capabilities for firefox like this:

{
  browserName: 'firefox',
  'moz:firefoxOptions': {
    args: ['--width=400', '--height=768']
   }
};

This works fine, opening firefox at the correct size. But I also use the wdio-firefox-profile-service to supply a profile and the firefox-profile-launch-service.js overwrites the 'moz:firefoxOptions' like this:

cap['moz:firefoxOptions'] = {
  profile: zippedProfile
}

If I change firefox-profile-launch-service.js to add the property instead it works:
cap['moz:firefoxOptions'].profile = zippedProfile;