Posting this here directly because it's so short and I don't feel like going to usertstyle's site:

@namespace url(http://www.w3.org/1999/xhtml);

@-moz-document url("https://www.fimfiction.net/feed") {
.feed {
max-width: 90%;
}
}

EDIT: Why does [code} remove linebreaks?

5983556
Try: [code=css] or, better yet, [codeblock=css]

5983556
Because inline code is useful and can't do that without 2 tags.