var IEpath = "C:\\Program Files\\Internet Explorer\\Iexplore.exe";
var file = Components.classes['@mozilla.org/file/local;1'].getService(Components.interfaces.nsILocalFile);
var process = Components.classes['@mozilla.org/process/util;1'].getService(Components.interfaces.nsIProcess); file.initWithPath(IEpath);process.init(file);
var arr=new Array();
arr[0]=window._content.location.href;
var out = {};
process.run(false,arr,arr.length,out);

昔、2chのスレに書いてあったやつ。