I love Violentmonkey. I use it to make the user names on HN larger so they are more prominent when I'm reading posts, as I don't agree with HN's design to deprioritize post authorship.
@-moz-document domain("news.ycombinator.com") {
.hnuser { font-size: 20px }
}
You'll have to restart Firefox for it to come into effect. .hnuser {font-size: 20px;}Being able to run normal JavaScript also makes it pretty easy to find and replace arbitrary elements - I use it for my personal Hacker News theme.
I could be mistaken but, the comment of the person to whom I was replying suggests they only need "username font size change" functionality. If that is all that is needed, then Stylus from a UI standpoint is far easier to manage.
But if in addition to merely changing font size, you're also going to be replacing elements, obviously Stylus can't do that.