break button disabled on firefox for some users

This seems to be a different issue from the other teaser break issues reported. Using ckeditor 3.0 on 6.x-2.4 wysiwyg. It's been working fine for some time. About a month ago, one user reported that she couldn't add a teaser break. I was unable to duplicate this, using her drupal account, so it seemed to be a browser issue. Just now, I am able to duplicate the problem. It is only occuring on firefox (9.0.1), both windows and linux versions. What is happening is that the break button is appearing, but it is disabled in firefox, i.e the class on the div is set to TB_Button_Disabled.

Doing the exact same thing in chrome, the button works, and the class appears as TB_Button_Off.

Nothing has changed on the drupal site during this period-- i.e. no software changes. Given that the problem is also not connected to the drupal account or software, it seems that the difference is entirely based on the browser being used. Presumably this is a CKEditor issue not a drupal one, but I'm reporting it here so we can track it, just in case, and because it will affect users of this module. I'm going to upgrade ckeditor and see if that fixes the problem.