The point is gather information on whether it's ready for a full scale rollout. The Firefox developers might know that it might be ready, but the sheer number

The point is gather information on whether it's ready for a full scale rollout. The Firefox developers might know that it might be ready, but the sheer number of software and hardware combinations out there might reveal unknown issues. So instead of rolling it out to 100% of targeted users, you roll it out to 1% of targeted users, and measure regressions to your metrics amongst that group.