Index: wp-includes/js/tinymce/tiny_mce_config.php
===================================================================
--- wp-includes/js/tinymce/tiny_mce_config.php	(revision 4929)
+++ wp-includes/js/tinymce/tiny_mce_config.php	(working copy)
@@ -74,6 +74,7 @@
 	convert_newlines_to_brs : false,
 	remove_linebreaks : false,
 	fix_list_elements : true,
+	gecko_spellcheck : true,
 	entities : "38,amp,60,lt,62,gt",
 	content_css : "<?php echo $mce_css; ?>",
 	valid_elements : "<?php echo $valid_elements; ?>",