What could cause this HTML/CSS rendering issue in Firefox?

Question

The styles:

h2 {
    color: #71D0FF;
    font-size: 11px;
    font-weight: bold;
    margin: 0px 0px 5px 0px;
}
a.box {
    color: #FFFFFF !important;
    cursor: pointer;
    display: block;
    padding: 10px;
    text-align: justify;
}
a.box:hover {
    background-color: #0C0C0C;
}
a.box span.down {
    display: block;
    color: #D04242;
    float: right;
    font-size: 11px;
    margin-left: 5px;
}
a.box span.up {
    display: block;
    color: #71D013;
    float: right;
    font-size: 11px;
}
span.noob {
    color: #FFA142;
}
span.pro {
    color: #A142A1;
}

The HTML (this is basically repeated for each link):

<a href="/library/blaze" class="box">
<span class="down">-0</span>
<span class="up">+0</span>
<h2><span class="noob">NOOB</span> BLAZE</h2>
HAS CREATED 0 MAPS, WON 0 BATTLES, AND LOST 0 MAPS
</a>

What I'm not understanding is why it renders differently in Firefox occasionally. Sometimes it shows up like it's supposed to and sometimes it shows up in this weird format as seen here:

I've never had anything like this happen before, does anyone know what's causing it? Does it even do this for anyone else? Like I stated before, sometimes it loads just fine (exact same HTML, CSS, and everything) and sometimes it doesn't. It seems kind of random. It loads just fine in IE with no weird problems at all.

Answer 1

Interesting. I can definitely reproduce it on FF3.6, actually I'm getting the broken version more often.

I can't get my head around it right now, but looking at it in Firefox, there is something broken with the link. If you open "inspect element" in Firebug, you will notice that the rendered DOM definitely changes between the intact and broken view. Firebug will also add _moz-rs-heading to the link, which is sort of explained here.

The first step should definitely be making the markup W3C valid and checking whether it still occurs.