I have a modified dom.push.userAgentID preference, type is "string" (I won't share the value, may be unique), in about:config. I certainly haven't modified or created this manually, but an extension may have created the preference.
1 Answer
Its the user agent used by dom.push (the WebSocket notification system introduced in Firefox 44) if privacy.resistFingerprinting is enabled (boolean true).
As for the "uniqueness" it seems to be just some random hex string. Resetting the preference will regenerate another random hex almost immediately.
