HTML formatting errors in FireFox - extra paragraphs

http://www.fckeditor.net/demo

<p><span style="color: rgb(51, 102, 255);"><span style="font-size: medium;">line one</span></span></p>
<p><span style="color: rgb(51, 102, 255);"><span style="font-size: medium;">line two</span></span></p>
<p><span style="color: rgb(51, 102, 255);"><span style="font-size: medium;">line three</span></span></p>

<p>&nbsp;</p>
<p><span style="color: rgb(255, 102, 0);">
<p><span style="font-size: medium;">line one</span></p>
</span></p>
<p>&nbsp;</p>
<p><span style="color: rgb(255, 102, 0);">
<p><span style="font-size: medium;">line two</span></p>
</span></p>
<p><span style="color: rgb(255, 102, 0);"><span style="font-size: medium;">line three</span></span></p>