How to detect Firefox mobile with javascript?

I'm using the following code to detect whether the browser being used on my mobile site matches a certain crieteria:

var isiPhone = navigator.userAgent.match(/iPhone/i) != null;
if (isiPhone){ alert ('iphone');

but if I attempt to do this for Firefox / Mozilla, I can't get it to work. I've tried:

var isFirefox = navigator.userAgent.match(/Mozilla/i != null);

and

var isFirefox = navigator.userAgent.match(/Firefox/i != null);

I visited whatismyuseragent.com and got the following:

Mozilla/5.0 (Android;Linux armv7l; rv6.0) Gecko/20110811 Gecko Firefox/6.0 Fennec/6.0

Any idea how I properly detect this? I need to write some firefox specific code.