Hello,
When in post editor mode, I cannot highlight the text. The gif below shows me trying to highlight the text, and then removing it. As you can see, it is not marked. So to clarify, it acts as if it is highlighted, but it does not show the text as highlighted.
I am now running Firefox version 23. Even with all extensions, this issue occurs. I believe that I have complaint about the editor previously, as it behaves differently than I expect when I want to regret (ctrl z).
GIF: http://tinypic.com/view.php?pic=nxmbfk&s=5