userContent.css does not work for Slack in Firefox

Question

I'm trying to make default Slack fonts bigger. I'm using Firefox's userContent.css:

@-moz-document domain(mastodon.social) {
    .column {
        flex: 1 0 25% !important;
    }
};

@-moz-document domain(slack.com) {
    [lang] body {
        font-size: 18px;
    }
    
    [lang] .c-message, [lang] #msg_input {
        font-size: 17px !important;
    }
}

The mastodon section works like a charm. The slack section, however, refuses to work, the userContent.css rules not appearing in rules lists for any given element.