 <p>You are not allowed to use block level elements like <code>&lt;div&gt;</code> inside of a <code>&lt;p&gt;</code>. Your browser fixes this mistake for you by ending the <code>&lt;p&gt;</code> before openeing the <code>&lt;div&gt;</code> - and Firebug displayes the fixed HTML.</p> <p>It would be wise to change your <code>&lt;p&gt;</code> to <code>&lt;div class="something"&gt;</code>.</p>