I had posted this when I was running version 2.3.3, and was told rather unkindly that this was a FireFox issue and that they should solve it. I had also found information that this error would not occur in 2.5, that it would be fixed.
Well, it does it now in both IE 7 and Firefox, when trying to add a new category both on the manage categories page and post page. So apparently this hasn't been resolved in 2.5. What are we to do?