Typo.

layout/xul/base/src/grid/nsGridRowGroupFrame.h

@@ -51,7 +51,7 @@
 /**
  * A frame representing a grid row (or column) group, which is usually
  * an element that is a child of a grid and contains all the rows (or
- * all the columns).  However, multiple level of groups are allowed, so
+ * all the columns).  However, multiple levels of groups are allowed, so
  * the parent or child could instead be another group.
  */
 class nsGridRowGroupFrame : public nsBoxFrame