Firefox OS is a new mobile phone operating system. which is open source and Linux based.
The ZTE Open is the first Firefox OS based phone from a major producer, and this is what I tested to see how different counters reported a visit to this website.

The user agent string is:
Mozilla/5.0 (Mobile; rv:18.0) Gecko/18.0 Firefox/18.0
which should identify as Firefox 18 / Firefox OS
Firefox / Unknown:
BraveNet
GoStats
24Log
SiteMeter
Firefox 18 / Unknown:
ExtremeTracking
ShinyStats
Firefox 18 / Firefox OS:
Statcounter
W3
Thus, currently, only two counters correctly identify Firefox OS ~ Statcounter, and W3.