Thanks zoonini, but I wouldn't know where to put:
define('CONCATENATE_SCRIPTS', false );
In your wp-config.php file.
If everything is fine in IE then you might want to look at upgrading Firefox to version 4 as it suggests in the troubleshooting list, assuming you're running version 3 now.
You should also try deactivating all plug-ins to see if there is a conflict. You shouldn't lose any changes anywhere if you do this. If turning off all your plug-ins solves the issue, then reactivate them one by one to find the culprit.
BTW - if you're using Thesis you might also want to ask for support over there as well, since that is a premium theme - they may have other suggestions.