rest – Accessing a node over core’s JSONAPI makes my browser try to download something

When I go to http://mysite/jsonapi/node/article/dfe90ec0-cd36-415c-bc36-ccc0ce6921f4, Firefox pops up saying I’m trying to open a ‘application/vnd.api+json’ and it wants to download it.

Why can’t I see the JSON in browser, the same way as with the core REST module?