If there are enough data points (which it seems like there is), and each UI is chosen a random 1/4 of the time, then it will be very easy to isolate and determine the effect of the slider UI. Problems would arise if the slider was chosen 60% of the time in Firefox, but only 30% of the time in Opera, and if Opera had a 20% higher population of hackers, who had a distribution centered around 6 as opposed to the aggregate population centered around 5; as is, it sounds like there's plenty of random sampling solving these complications.