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That page is quite outdated (2007), a better reference for about:config referrer options is https://wiki.mozilla.org/Security/Referrer https://wiki.mozilla.org/
by LukeShu 9y ago
That page is quite outdated (2007), a better reference for about:config referrer options is https://wiki.mozilla.org/Security/Referrer https://wiki.mozilla.org/Security/Referrer (which is linked to at the bottom of the blog post about FF59 being discussed).
It seems that Firefox 59 will effectively force about:config's network.http.referer.XOriginTrimmingPolicy to 2 (default is 0) when in private browsing.