I *think* you can use netscape or mozilla on the older mac os. And I believe those browsers would be better than the mac version of IE. I rarely use Macs, so I can't be completely sure, but it's worth a shot.
VB, the people that make the forum software, do not support IE on Mac for this latest release (apparently).
Another thought I had: If you take a look in your user cp in the forum, you have an option in there for different ways to edit posts. I wonder if you switch from the WYSIWYG editor to the older method if you'd have the same problem. It's under "edit options" and you have a choice of: enhanced (wysiwyg), the standard or basic editors.