Not just for Firefox OS but for any Firefox user...

Firefox Hello allows you to have ad-hoc or planned audio/video conversations with anyone using any browser that supports WebRTC: currently, this includes latest versions of Firefox, Chrome and Opera.

It's incredibly simple to set up - just click the chat button in Firefox, and click 'start a conversation', then share the URL with whoever you wish to speak with - there's a button for emailing the link or copying it to your clipboard. They click the link to open in Firefox, Chrome or Opera (other browsers coming soon). It's that easy. The URL can be persistent so, once it is in place, you can chat any time and invite anyone to join you. 

Unlike incumbents like Microsoft Skype, Google Hangouts or Apple FaceTime, there is no need to share private information with a company intent on mining your data, no sign-up, no conversations visible to national security organizations (well - at least it is nothing like as easy for them to track and record). Once set up, calls go directly between browsers, without stopping along the way on someone else's server. If you would like a more persistent link, it is possible to sign in to your Firefox.com account, in which case you can manage contacts and connect with others having an account much as you would in Hangouts or Skype.

The main thing it currently lacks is good notification support - although, if you have Firefox open, you will get an alert when someone tries to join one of your URLs (assuming you have signalled you are available), it is not as trivial as Skype or FaceTime to call someone ad hoc. It's more like Google Hangouts on the desktop, which requires you to have Chrome open to receive calls. On the bright side, Firefox is now a much nicer browser than Chrome, and far less resource-hungry, so it is not such a bad thing.

A minor inconvenience is that, once a conversation is running, it takes over your webcam, whether or not you have it switched on for the conversation. I'd call that a bug rather than a feature, and hope it will be fixed soon.

It is also voice and/or video only - no text chat, screen sharing, file sharing or other useful extras yet. But it's early days. I'm guessing things like this will come in later releases.