Firefox: navigator.getUserMedia is not a function

It's not supported unprefixed yet. See http://caniuse.com/#search=getusermedia

You'll need to get the browser-specific prefix and use that. As posted in another answer:

navigator.getUserMedia = ( navigator.getUserMedia ||
                       navigator.webkitGetUserMedia ||
                       navigator.mozGetUserMedia ||
                       navigator.msGetUserMedia);

This will set navigator.getUserMedia to whatever it detects to be the proper prefixed version.

Since navigator.getUserMedia() is deprecated, use this :

navigator.getUserMedia = (
    navigator.getUserMedia ||
    navigator.webkitGetUserMedia ||
    navigator.mozGetUserMedia ||
    navigator.msGetUserMedia
);

if (typeof navigator.mediaDevices.getUserMedia === 'undefined') {
    navigator.getUserMedia({
        audio: true
    }, streamHandler, errorHandler);
} else {
    navigator.mediaDevices.getUserMedia({
        audio: true
    }).then(streamHandler).catch(errorHandler);
}

Check the MDN page, especially the first part of the first example:

navigator.getUserMedia = ( navigator.getUserMedia ||
                       navigator.webkitGetUserMedia ||
                       navigator.mozGetUserMedia ||
                       navigator.msGetUserMedia);

It's not that well-supported - you'll need browser prefixes.