My form views fine in IE7 and IE8 but FireFox does not display the form correctly: The problem is it does not display the form inside my "mainContent1"

Note my code below:

<div id="mainContent1">
<form action="forms.php" target="_self">
<fieldset>
<legend>Postal Address</legend>  
<label for="street">Street address:</label>
<input id="street" name="street" type="text" />
<label for=" suburb">County</label>
<input id="county" name="county" type="text" />
<label for="state">State</label>
<input id="state" name="state" type="text" />
<label for="zip">Zip Code</label>
<input id="zip" name="zip" type="text" />
</fieldset>
</form>
</div>