Textarea在光标停留处插入文字

<!-- function Insert(str) { var obj = document.getElementById('content'); if(document.selection) { obj.focus(); var sel=document.selection.createRange(); document.selection.empty(); sel.text = str; } else { var prefix, main, suffix; prefix = obj.value.substring(0, obj.selectionStart); main = obj.value.substring(obj.selectionStart, obj.selectionEnd); suffix = obj.value.substring(obj.selectionEnd); obj.value = prefix + str + suffix; } obj.focus(); } --> 

?兼容IE、firefox