Index: wp-includes/class-wp-editor.php
===================================================================
--- wp-includes/class-wp-editor.php	(revision 22127)
+++ wp-includes/class-wp-editor.php	(working copy)
@@ -608,7 +608,13 @@
 						try { tinymce.init(init); } catch(e){}
 				}
 			} else {
-				el = document.getElementsByClassName('wp-editor-wrap');
+				if( document.getElementsByClassName ) {
+					el = document.getElementsByClassName('wp-editor-wrap');
+				}
+				else {
+					el = document.querySelectorAll('.wp-editor-wrap');
+				}
+
 				for ( i in el ) {
 					if ( typeof(el[i]) == 'object' )
 						el[i].onmousedown = function(){ wpActiveEditor = this.id.slice(3, -5); }