Cookie law related, perhaps? There was a cookie-related change in FF43 for third-party cookies (<a href="https:&#x2F;&#x2F;bugzilla.mozilla.org&#x2F;show_bug.cgi?id=536509" rel="nofollow">https:&#x2F;&#x2F;bugzilla.mozilla.org&#x2F;show_bug.cgi?id=536509</a>).