The first time you start Firefox, it gives you a notification (bottom bar, I think) among the lines of "we're collecting stats for improvement" and two buttons "Ok" and "Disable". IIRC, it defaulted to enabled, but didn't appear sneaky to me at all.
If anything, I am interested in the bias this data will have since I suspect many advanced users instinctively opt out of data collection policies. I opt out of anything that I can, and I would opt out of more if the options were readily available. Since advanced users are more likely—in my estimation—to have higher-specification hardware, I suspect the data is slightly biased toward the low-end.
(I more or less instinctively opt out of data collection and have never enabled it in Firefox.)
Still, the self selection bias is a curiosity.