It's just another database collecting unknown information about me ("anonymized" in some way that may be reversible), stored for an unknown length of time, and enabled by default. Just ask. Plenty of people will beta test software for a $20 gift certificate, or even for free, but they should be given a choice.
Do they? I seem to remember a great kerfuffle somewhat recently over telemetry being opt-out.
Studies _must_ be opt-out given the amount of users Mozilla says the fix covers, and they're basically a form of telemetry, in their intended use anyway.
