Mozilla has decided to delete all telemetry, or user’s usage and performance data, that was collected by Firefox while they were rolling out the study to fix disabled Firefox addons. For many people, this was a problem as you needed to allow usage data to be uploaded to Mozilla in order to enable the studies. Mozilla released Firefox 66.0.4 and 66.5 to resolve the issues with the issue without the need for a study, but for many it was too late as they had already enabled studies.
Source: https://www.bleepingcomputer.com/news/software/mozilla-to-delete-usage-data-collected-from-firefox-addon-fix/