I just edited one of my own messages, and noticed that the cursor was gone. I clicked the text, but got no cursor in the editorbox whatsoever. Nothing I typed appeared. Pressing tab moved the focus back and forth, so the focus must have been on the text editor box, but I could not type. Happened 2 out of 4 times, so not always.
Firefox 3.6.13, WinXP
Could not reproduce it with Chrome. (Interestingly enough, I cannot reproduce it with Firefox either now after a few minutes. It happened right after I entered my post.)