Index: wp-includes/general-template.php
===================================================================
--- wp-includes/general-template.php	(revision 19896)
+++ wp-includes/general-template.php	(working copy)
@@ -1759,6 +1759,11 @@
 				$wp_rich_edit = true;
 			}
 		}
+
+		// Kindle Fire / Amazon Silk - doesn't support `contentEditable`
+		if( false !== strpos( $_SERVER['HTTP_USER_AGENT'], '; Silk/' ) )
+			$wp_rich_edit = false;
+
 	}
 
 	return apply_filters('user_can_richedit', $wp_rich_edit);