When entering 'fanfiction.net' or 'https://fanfiction.net', domain-for-sale sites appear. However, on Safari, it correctly redirects to 'https://www.fanfiction.net'
Hm, https://fanfiction.net/ is responding with a 403 Forbidden. If the server doesn't respond with a redirect, then we surely should not create one ourselves? How are we supposed to know that it wants to redirect to www.fanfiction.net if it doesn't tell us to? Most websites will redirect www.foo.com to foo.com or vice-versa, but that's just convention and certainly not required.
You're right, no other browser seems to do this...