After upgrading to 4.6 CiviMail HTML version is uneditable
There's an issue+patch filed for 4.6.1 at https://issues.civicrm.org/jira/browse/CRM-16246. You can try disabling Drupal's aggregation as a work-around (by visiting "admin/config/development/performance"). If that work-around doesn't resolve it, let us know (since that would mean it's a different problem).