Firefox has telemetry, it just doesn't gather data on this feature. It does gather: Interaction data includes information about your interactions with Firefo

Firefox has telemetry, it just doesn't gather data on this feature. It does gather: Interaction data includes information about your interactions with Firefox such as number of open tabs and windows, number of webpages visited, number and type of installed Firefox Add-ons and session length, as well as Firefox features offered by Mozilla or our partners such as interaction with Firefox search features and search partner referrals. [0] I don't think telling if the compact UI was enabled is more invasive than this data. [0] https://support.mozilla.org/en-US/kb/telemetry-clientid https://support.mozilla.org/en-US/kb/telemetry-clientid