Stackoverflow answered a question in late 2016 about how, specifically, to fingerprint Private Browsing users for whatever reason: https://stackoverflow.com/questions/2860879/detecting-if-a-browser-is-using-private-browsing-mode/37091058 Which spawned this gist, that is being maintained actively to this day: https://gist.github.com/jherax/a81c8c132d09cc354a0e2cb911841ff1#file-is-private-mode-js-L32 While the SO question implies that it's to _require_ private browsing mode, sites that depend on user tracking revenue in the wild are using the same logic to refuse service to Private Browsing users.
Bug 781982 Comment 76 Edit History
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Stackoverflow answered a question in late 2016 about how, specifically, to fingerprint Private Browsing users for whatever reason: https://stackoverflow.com/questions/2860879/detecting-if-a-browser-is-using-private-browsing-mode/37091058 Which spawned this gist, that is being maintained actively to this day: https://gist.github.com/jherax/a81c8c132d09cc354a0e2cb911841ff1#file-is-private-mode-js-L32 // Firefox if ('MozAppearance' in document.documentElement.style) { const db = indexedDB.open('test'); db.onerror = on; db.onsuccess = off; return void 0; } While the SO question implies that it's to _require_ private browsing mode, sites that depend on user tracking revenue in the wild are using the same logic to refuse service to Private Browsing users.