[ConfusedTA]

If you're using a browser which support Stylish (which I'm assuming you do, having FireBug installed) - link here for Stylish for Firefox.

Install Stylish, click on the icon, and select "Write a new style", and copy/paste the below code into the box it gives you.

Code:

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

@-moz-document domain("forum.xbmc.org") {

    #container {
        margin: 0 auto;
        padding-top: 3em;
        max-width: 900px;
    }
}