FF adds excessive <br> tags on enter

[Damian]

Steps to reproduce:

1. Open nightly build (3275 as of writing)
   
2. Add text followed by Enter key a number of times e.g. (a ENTER b ENTER c ENTER)
   
3. Examine HTML
   

Expected behavior:
New lines are wrapped with a <div> only

Actual result:
Firefox only: Excessive <br/> tags are appended within the dom e.g.:

<p>
	a<br />
	<br />
</p>
<p>
	b<br />
</p>
<p>
	c<br />
</p>

[Garry Yao]

Duplicate of #2886.