In FF3, when multiple paragraphs are selected and the text is changed (color, font, size, highlighted), the paragraphs are dropped down a line and a new <p> is inserted before the paragraphs.
Duplication Steps:
1. Go to http://cutesoft.net/example/oncarriagereturn.aspx and use <p> as the line break
2. In normal view, type anything then press <enter> then type anything
3. Click to the right of the last line causing it to show <p> on the bottom of the editor
4. Select all text with the mouse and change the font (or color, size, or highlight)
5. Change the view to HTML - notice <p> </p> has been added to the top of the text
Output:
<p> </p>
<span style="font-family: Comic Sans MS;">
<p>asd</p>
<p>asd </p>
</span>
<p><br />
</p>