When entered directly, Reader Mode did not strip the username and password section of URLs displayed in the addressbar. This can be used for spoofing the domain of the current page. This vulnerability affects Firefox < 54.
https://www.mozilla.org/security/advisories/mfsa2017-15/
https://bugzilla.mozilla.org/show_bug.cgi?id=1358248
https://access.redhat.com/errata/RHSA-2018:2113
https://access.redhat.com/errata/RHSA-2018:2112