Follow these steps:
1. Type asd
2. Hit enter
3. Hit the paragraph button 3x
In firefox I have this markup created when I do that:
<p>asd</p>
<p> </p>
<p> </p>
<p> </p>
<p> </p>
In IE I have this markup created instead:
<p>asd</p>
<p>
<p> </p>
<p> </p>
<p> </p>
</p>