Well, the https://testpilot.firefox.com/experiments https://testpilot.firefox.com/experiments page shows the number of people participating in each experiment.

Well, the https://testpilot.firefox.com/experiments https://testpilot.firefox.com/experiments page shows the number of people participating in each experiment. Currently, Activity Stream has the most participants with 36,056⁩; No More 404s is currently showing 2,468⁩.