Thunderbird and atom feed: HTML displayed as raw text

So I have this atom feed at https://what-if.xkcd.com/feed.atom that I would like Thunderbird to read from. But it shows up as raw html.

Background is how content displays, window on top is source brought up by "view source" (click for screenshot)

Not a huge problem since this website is inactive anyway (maybe the issue even is the server side atom stuff) but I still would like to know how to fix the issue if it ever happens for useful feeds.