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Using Firefox 16.0a1 (2012-06-07) on OS X it's telling me my battery is at 100% and charging, while it's at 95% and not charging.

From the article "And it doesn't work yet on Mac (does work on iOS, Android and windows)."

Didn't work for me on Android 4.0.3 in either Chrome or the Android browser

I suspect it might only be in the Firefox beta stream? (In which case, I'm surprised to see the OP mention iOS).