Hallo,
ich habe die Information mittlerweile auf Stackoverflow bekommen:
http://stackoverflow.com/a/20105070/1692735
Short answer:
CodeMozilla/5.0 (Mobile; ZTEOPEN; rv:18.1) Gecko/18.1 Firefox/18.1Also, if you want to know the LG Fireweb UA string, it is:
CodeMozilla/5.0 (Mobile; LG-D300; rv:18.1) Gecko/18.1 Firefox/18.1
Long answerThe Keon UA string is actually the Mozilla's recommended UA string for all Firefox OS smartphones. See the gecko user agent string reference, which means that you can be serving the simplified version of your website for devices with lots of RAM in the future, devices that would use the recommended string and that are not Geeksphone Keon, a Nexus 4 running Firefox OS for example.
And that is the trouble of using user agent sniffing. That practice should be avoided whenever possible because of it's fragility as you may already know… relying on UA sniffing instead of feature detection is not a good practice.
There are some ways to test the performance of your page without trying to guess which device/platform/version your visitor uses, RequestAnimationFrame can be an alternative way to probe the actual responsiveness of your app for example, but I don't know if that would be enough for your usecase.
Vielen Dank! 
SunboX