Although, in theory, WebRTC should work on any device Firefox for Android supports, for an ideal experience you're going to want to have an Ice Cream Sandwich or Jelly Bean device. We'll do what we can for older devices, but realistically Android was missing some essential APIs before that time (especially before Android 2.3) and device performance will also limit what experience you get. Nevertheless I had no problems getting a video call up on my Galaxy Tab 10.1 with Android 3.2, and many devices will probably work just fine.
By default WebRTC is included in Firefox for Android, but behind a preference that is off by default. To enable, flip these preferences to "true":
Secondly, because the UI is currently very provisional, you might want to disable the dialog that asks for permission to use your camera and microphone (but remember the implications of this, you probably don't want to keep it that way after testing!). Enable this setting:
Some example pages for WebRTC APIs are here:
Note that as the code only just landed, there are going to be bugs. Some known ones are the permissions dialog (if enabled) only remembering the first choice you make, giving you audio or video, but not both, and when disabling permissions, we always take the first camera, which is probably the one you didn't want. On some devices the video might be upside down, too. Expect most of these to be fixed in the next days and weeks. If you find any bugs that I haven't listed here, feel free to file in Bugzilla and we'll get to work on them.