Ephera OP , 2 months ago

Well, making such a change permanent on a foreign webpage, that's not really something you do in webdev, but I happen to know anyways.
What you want is usually referred to as a "user style" and you can achieve that with e.g. the Stylus extension.

You can create a new style and then paste this into it:

@-moz-document domain("fandom.com") {
    .global-navigation {
        display: none;
    }

    body {
        margin-left: 0 !important;
    }
}

Then save it and fandom.com should look nicer.

If you ignore the first and the last line, these are normal CSS rules, so that is then where webdev knowledge can help...