Hello Adam,
I'm using the Firefox 1.5.0.1 version and Windows XP. A friend also confirmed the bug using Firefox on a Mac (not sure about the version).
Yes it can be reproduced on the online demo.
If you insert a <br> in front of the table on the online demo, you will se the same problem. On the online demo you will have to insert it in html-mode. When saving after inserting the <br> and returning to Normal mode, everything within the editor field is gone.
In our application the "blank line" is represented by <div> </div> in Internet Explorer and by <br /> in Firefox.
tomsl