In Reader View SameSite cookie protections are not checked on exiting. This allows for a payload to be triggered when Reader View is exited if loaded by a malicious site while Reader mode is active, bypassing CSRF protections. This vulnerability affects Firefox < 61.
https://www.mozilla.org/security/advisories/mfsa2018-15/
https://usn.ubuntu.com/3705-1/
https://security.gentoo.org/glsa/201810-01