I can tell you that we test in Firefox 4.x and 3.6, Google Chrome (latest), Apple Safari (latest), Opera (latest) on both Windows and Mac, Internet Explorer versons 8, 7 and 6, and with Safari under iOS.
To further that I have seen a few times when I was editing / creating posts the flashing vertical line disappear and have to highlight some text back to the point I wanted to edit and hit the left arrow to know exactly where I was typing, and after that the flashing line in the editor notating where I am typing still wont reload and just have to trust that it's there. It's not often but once in a while there is some erroneous behavior. Mozilla/5.0 (Windows; U; Windows NT 6.1; en-US; rv:220.127.116.11) Gecko/20100722 Firefox/3.6.8
Ahh I'm a noob i realized i posted from the wrong instance of FF Disregard this comment.