In private browsing mode on Firefox for Android, favicons are cached in the cache/icons folder as they are in non-private mode. This allows information leakage of sites visited during private browsing sessions. *Note: this issue only affects Firefox for Android. Desktop versions of Firefox are unaffected.*. This vulnerability affects Firefox < 63.
https://www.mozilla.org/security/advisories/mfsa2018-26/
https://bugzilla.mozilla.org/show_bug.cgi?id=1448305