--- a/layout/style/ServoStyleSet.cpp
+++ b/layout/style/ServoStyleSet.cpp
@@ -655,16 +655,17 @@ ServoStyleSet::ReplaceSheets(SheetType a
 nsresult
 ServoStyleSet::InsertStyleSheetBefore(SheetType aType,
                                       ServoStyleSheet* aNewSheet,
                                       ServoStyleSheet* aReferenceSheet)
 {
   MOZ_ASSERT(aNewSheet);
   MOZ_ASSERT(aReferenceSheet);
   MOZ_ASSERT(aNewSheet->IsApplicable());
+  MOZ_ASSERT(aNewSheet != aReferenceSheet, "Can't place sheet before itself.");
   MOZ_ASSERT(aNewSheet->RawSheet(), "Raw sheet should be in place before insertion.");
   MOZ_ASSERT(aReferenceSheet->RawSheet(), "Reference sheet should have a raw sheet.");
 
   uint32_t beforeUniqueID = FindSheetOfType(aType, aReferenceSheet);
   if (beforeUniqueID == 0) {
     return NS_ERROR_INVALID_ARG;
   }
 

--- a/layout/style/StyleSetHandleInlines.h
+++ b/layout/style/StyleSetHandleInlines.h
@@ -180,17 +180,17 @@ StyleSetHandle::Ptr::ReplaceSheets(Sheet
 nsresult
 StyleSetHandle::Ptr::InsertStyleSheetBefore(SheetType aType,
                                 StyleSheet* aNewSheet,
                                 StyleSheet* aReferenceSheet)
 {
   FORWARD_CONCRETE(
     InsertStyleSheetBefore,
     (aType, aNewSheet->AsGecko(), aReferenceSheet->AsGecko()),
-    (aType, aReferenceSheet->AsServo(), aReferenceSheet->AsServo()));
+    (aType, aNewSheet->AsServo(), aReferenceSheet->AsServo()));
 }
 
 int32_t
 StyleSetHandle::Ptr::SheetCount(SheetType aType) const
 {
   FORWARD(SheetCount, (aType));
 }