Did I tell you about the time I inadvertently locked out an early-adopter of MSIE 10? Yup: code said MSIE [1-4] without trailing \. or \b. If they're using ::yawn:: UA detection rather than feature detection, they could easily have something that says "Firefox " left over from, uh, 2011 or whenever FF5 was released.
:: shuffling papers ::
Oh, ###. I meant that as a joke, but quick detour to FF Updates List reveals that my own computer went to FF5 in mid-August 2011, followed by FF6 scarcely a week later.