Exactly. It's easy to opt out, as long as you're paying a bit of attention when you first start up Firefox. It's easy to opt-out later if you are comfortable navigating the preferences dialogs.

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.