As we have posted in the past, the active daily user counts we report in AMO statistics dashboards are always slightly inflated, but during Firefox releases they can be inflated much higher. Active daily user counts are just the number of update pings we get for each add-on in a day, but until Firefox 3.6, we had no way of knowing whether they were daily background update checks, manual checks, checks when upgrading Firefox, checks when an add-on is incompatible, etc. This is what causes the count inflation: some users are counted more than once in the same day.
Firefox 3.6 added a new parameter to the update check that tells us the type of check. This allows us to start counting only the daily background checks instead of all checks, leading to more accurate user counts and less volatility.
We plan to switch over to this new counting method on Monday, March 1. If an update ping has the update type information, we will only count that ping if it is a daily background check. There will still be some inflation, as update pings that don’t have the type information (versions before Firefox 3.6) will continue to be counted. Starting Monday night, add-on developers should expect active user counts to drop up to 20%, depending on how many of the add-on’s users are on Firefox 3.6.
We hope this will help developers be more confident in these statistics going forward, and look forward to users continuing to upgrade to Firefox 3.6 so that we can eliminate inflation entirely.