ADI stands for "Active Daily Installations." ("ADU" or "Active Daily Users" is sometimes used to refer to the same number.) Mozilla measures Firefox usage by the number of Firefox installations that retrieve blocklist updates from Mozilla's servers each day. This measures the number of computers running Firefox that day (excluding any that have the blocklist disabled or cannot connect to the update server for some reason).
The number of Active Daily Installations is less than the total number of Firefox users in the world, since many users do not run their browser every day. (And some users run multiple Firefox installations, perhaps on multiple devices. Each of those installations will count as a separate ADI.)