RSS feeds from Tumblr (http://subdomain.tumblr.com/rss) are not accepted by Thunderbird (version, 52.8.0; feed invalid). The validity check of these feeds returns with a passed validity check. The feed can be subscribed to without any problem using Firefox. Disabling Lightning or blanking calendar.useragent.extra had no effect. The problem is present for multiple weeks uneffected by the latest Thunderbird updates.