Basically what I have is this:
<body>
<div class="class1">
</div>
<div class="class2">
</div>
<div class="class3">
</div>
...
</body>
I have no idea why the site creator used classes instead of IDs (they're unique), but it doesn't really matter as I'm writing a GM script and so getElementsByClassName('')[0] effectively does the same thing.
How can I insert an element between the first and second div?