Adam,
I am using latest version 6.6. Even though I am able to reproduce in the given example in Firefox 3.6. Steps to reproduce:
1. Click on Css Class dropdown and it opens.
2. Click on the editor text area(in which we write).
3. It should close the dropdown if clicks outside the dropdown(works fine in IE 8) but it is now closing the dropdown.
please let me know if you are not able to reproduce.
Sanjay