Mar

I'm really sorry but when I read the post title I understood that you are detecting a FirefoxOS system from among all systems, because behavior is not always the same.

Well, If someone else had my same confusion and really want to discriminate between a FirefoxOS system and anything else, I want to recommend changing my previous condition with this one. (!!navigator.mozId).

If you try it on a Mozilla Firefox navigator on an Android phone It returns true.
I also add (navigator.userAgent.search("Android")<0) into the condition to solve it.