Firefox desktop · Research guidance. Instructions are based on publisher guidance; a UK IPTV account was not tested on this platform.
The labels describe different jobs
Mozilla’s current Disable Firefox Sync article says disconnecting Sync stops synchronisation while leaving the account signed in and the profile’s data and settings in place. That is useful when you want a browser to stop exchanging information. It is a limited outcome, however, when the actual goal is handing a computer to another person.
Local removal is another choice
Mozilla separately documents an optional Delete data from this device choice during account sign-out. Its stated scope is the current device, not deleting information from other synced devices. Read the prompt actually presented and consider whether information you need has a legitimate retained copy before choosing a destructive cleanup option.
Even that list has boundaries
Mozilla lists items the sign-out cleanup does not remove, including website permissions and modified preferences. A person may therefore find a familiar site permission still configured after synced information has been removed. That observation does not prove sync remained active; it can reflect the documented difference between data classes.
Match the handover to the task
For a temporary pause in syncing, inspect the Syncing status. For account sign-out, inspect the account state. For a shared-device handover, also consider actual local files, website sessions and the computer user account. Describing those checks separately is more reliable than assuming one browser button completes every privacy task.
Questions, answered
Does Sync off mean all local data is gone?
No. Mozilla says it does not delete data or settings.
Does the optional device cleanup erase other synced devices?
No. Its documented scope is the current device.