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.

I consider myself an advanced user and I enable sending Telemetry and crash reports on Firefox explicitly. I dont do that on Chrome but I prefer doing that on Firefox. The point I am trying to make is, although I agree that some users may opt-out, many trust Mozilla and so would usually let it collect data like this.

A good point. If there were one major organization that I feel comfortable sending telemetry data to, it may be Mozilla—the only organization that makes respect for their user's privacy a paramount concern.

(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.

Even if you don't trust Mozilla, the list of things Firefox collects as telemetry on release is extremely limited. (You will have to trust that the binary you downloaded is built from source though, but you can build yourself and/or turn the pref off of you care that much).