add: document.getElementById("attachment-splitter").collapsed = true; 
context: function HideMessageHeaderPane(){  document.getElementById('msgHeaderView').collapsed = true;  // disable the File/Attachments menuitem  document.getElementById("fileAttachmentMenu").setAttribute("disabled", "true");  // disable the attachment box  document.getElementById("attachmentView").collapsed = true;}
meta: 10234	/local1/tlutelli/issta_data/temp/all_js/javascript/2007_temp/2007/10234/b031f74b44f2a7c8bdd4ee14d38c2bd7dd70ff60/msgHdrViewOverlay.js/clean/mail/base/content/msgHdrViewOverlay.js