diff --git a/layout/base/nsCSSFrameConstructor.cpp b/layout/base/nsCSSFrameConstructor.cpp
index 34886f063495..56dfb9f7c7de 100644
--- a/layout/base/nsCSSFrameConstructor.cpp
+++ b/layout/base/nsCSSFrameConstructor.cpp
@@ -340,6 +340,20 @@ IsFlexOrGridContainer(const nsIFrame* aFrame)
          t == nsGkAtoms::gridContainerFrame;
 }
 
+// Returns true IFF the given nsIFrame is a nsFlexContainerFrame and
+// represents a -webkit-{inline-}box container. The frame's GetType() result is
+// passed as a separate argument, since in most cases, the caller will have
+// looked it up already, and we'd rather not make redundant calls to the
+// virtual GetType() method.
+static inline bool
+IsFlexContainerForLegacyBox(const nsIFrame* aFrame,
+                            const nsIAtom* aFrameType)
+{
+  MOZ_ASSERT(aFrame->GetType() == aFrameType, "wrong aFrameType was passed");
+  return aFrameType == nsGkAtoms::flexContainerFrame &&
+    aFrame->HasAnyStateBits(NS_STATE_FLEX_IS_LEGACY_WEBKIT_BOX);
+}
+
 #if DEBUG
 static void
 AssertAnonymousFlexOrGridItemParent(const nsIFrame* aChild,
@@ -9921,13 +9935,17 @@ nsCSSFrameConstructor::CreateNeededAnonFlexOrGridItems(
   FrameConstructionItemList& aItems,
   nsIFrame* aParentFrame)
 {
-  if (aItems.IsEmpty() ||
-      !::IsFlexOrGridContainer(aParentFrame)) {
+  if (aItems.IsEmpty()) {
+    return;
+  }
+  const nsIAtom* parentType = aParentFrame->GetType();
+  if (parentType != nsGkAtoms::flexContainerFrame &&
+      parentType != nsGkAtoms::gridContainerFrame) {
     return;
   }
 
-  const bool isWebkitBox = nsFlexContainerFrame::IsLegacyBox(aParentFrame);
-
+  const bool isWebkitBox = IsFlexContainerForLegacyBox(aParentFrame,
+                                                       parentType);
   FCItemIterator iter(aItems);
   do {
     // Advance iter past children that don't want to be wrapped
@@ -12219,12 +12237,14 @@ nsCSSFrameConstructor::WipeContainingBlock(nsFrameConstructorState& aState,
   // Situation #2 is a flex or grid container frame into which we're inserting
   // new inline non-replaced children, adjacent to an existing anonymous
   // flex or grid item.
-  if (::IsFlexOrGridContainer(aFrame)) {
+  nsIAtom* frameType = aFrame->GetType();
+  if (frameType == nsGkAtoms::flexContainerFrame ||
+      frameType == nsGkAtoms::gridContainerFrame) {
     FCItemIterator iter(aItems);
 
     // Check if we're adding to-be-wrapped content right *after* an existing
     // anonymous flex or grid item (which would need to absorb this content).
-    const bool isWebkitBox = nsFlexContainerFrame::IsLegacyBox(aFrame);
+    const bool isWebkitBox = IsFlexContainerForLegacyBox(aFrame, frameType);
     if (aPrevSibling && IsAnonymousFlexOrGridItem(aPrevSibling) &&
         iter.item().NeedsAnonFlexOrGridItem(aState, isWebkitBox)) {
       RecreateFramesForContent(aFrame->GetContent(), true,
@@ -12264,7 +12284,8 @@ nsCSSFrameConstructor::WipeContainingBlock(nsFrameConstructorState& aState,
     // Skip over things that _do_ need an anonymous flex item, because
     // they're perfectly happy to go here -- they won't cause a reframe.
     nsIFrame* containerFrame = aFrame->GetParent();
-    const bool isWebkitBox = nsFlexContainerFrame::IsLegacyBox(containerFrame);
+    const bool isWebkitBox =
+      IsFlexContainerForLegacyBox(containerFrame, containerFrame->GetType());
     if (!iter.SkipItemsThatNeedAnonFlexOrGridItem(aState,
                                                   isWebkitBox)) {
       // We hit something that _doesn't_ need an anonymous flex item!
@@ -12288,7 +12309,6 @@ nsCSSFrameConstructor::WipeContainingBlock(nsFrameConstructorState& aState,
   //    their sibling changes.
   // 2) The first effective child of a ruby frame must always be a ruby
   //    base container. It should be created or destroyed accordingly.
-  nsIAtom* frameType = aFrame->GetType();
   if (IsRubyPseudo(aFrame) ||
       frameType == nsGkAtoms::rubyFrame ||
       RubyUtils::IsRubyContainerBox(frameType)) {
diff --git a/layout/generic/nsFlexContainerFrame.cpp b/layout/generic/nsFlexContainerFrame.cpp
index 9706ce9d5e98..136045b9a26d 100644
--- a/layout/generic/nsFlexContainerFrame.cpp
+++ b/layout/generic/nsFlexContainerFrame.cpp
@@ -87,10 +87,22 @@ IsDisplayValueLegacyBox(const nsStyleDisplay* aStyleDisp)
     aStyleDisp->mDisplay == mozilla::StyleDisplay::WebkitInlineBox;
 }
 
+// Returns true if aFlexContainer is the frame for an element with
+// "display:-webkit-box" or "display:-webkit-inline-box". aFlexContainer is
+// expected to be an instance of nsFlexContainerFrame (enforced with an assert);
+// otherwise, this function's state-bit-check here is bogus.
+static bool
+IsLegacyBox(const nsIFrame* aFlexContainer)
+{
+  MOZ_ASSERT(aFlexContainer->GetType() == nsGkAtoms::flexContainerFrame,
+             "only flex containers may be passed to this function");
+  return aFlexContainer->HasAnyStateBits(NS_STATE_FLEX_IS_LEGACY_WEBKIT_BOX);
+}
+
 // XXXdholbert This will be merged into Init(), in a later patch in this series
 // (after all callers have been converted to check frame state bit).
-/* static */ bool
-nsFlexContainerFrame::IsLegacyBox(const nsIFrame* aFrame)
+static bool
+IsLegacyBoxFOLD_ME(const nsIFrame* aFrame)
 {
   nsStyleContext* styleContext = aFrame->StyleContext();
   const nsStyleDisplay* styleDisp = styleContext->StyleDisplay();
@@ -1133,7 +1145,7 @@ IsOrderLEQWithDOMFallback(nsIFrame* aFrame1,
     return true;
   }
 
-  bool isInLegacyBox = nsFlexContainerFrame::IsLegacyBox(aFrame1->GetParent());
+  const bool isInLegacyBox = IsLegacyBox(aFrame1->GetParent());
 
   int32_t order1 = GetOrderOrBoxOrdinalGroup(aFrame1, isInLegacyBox);
   int32_t order2 = GetOrderOrBoxOrdinalGroup(aFrame2, isInLegacyBox);
@@ -1211,7 +1223,7 @@ IsOrderLEQ(nsIFrame* aFrame1,
     return true;
   }
 
-  bool isInLegacyBox = nsFlexContainerFrame::IsLegacyBox(aFrame1->GetParent());
+  const bool isInLegacyBox = IsLegacyBox(aFrame1->GetParent());
 
   int32_t order1 = GetOrderOrBoxOrdinalGroup(aFrame1, isInLegacyBox);
   int32_t order2 = GetOrderOrBoxOrdinalGroup(aFrame2, isInLegacyBox);
@@ -2284,7 +2296,7 @@ nsFlexContainerFrame::Init(nsIContent*       aContent,
 {
   nsContainerFrame::Init(aContent, aParent, aPrevInFlow);
 
-  if (nsFlexContainerFrame::IsLegacyBox(this)) {
+  if (IsLegacyBoxFOLD_ME(this)) {
     // Toggle frame state bit to indicate that this frame represents a
     // legacy -webkit-{inline-}box container:
     AddStateBits(NS_STATE_FLEX_IS_LEGACY_WEBKIT_BOX);
diff --git a/layout/generic/nsFlexContainerFrame.h b/layout/generic/nsFlexContainerFrame.h
index 49260589fe9d..b3fb9e90f2db 100644
--- a/layout/generic/nsFlexContainerFrame.h
+++ b/layout/generic/nsFlexContainerFrame.h
@@ -111,12 +111,6 @@ public:
                                     uint32_t* aNumPackingSpacesRemaining,
                                     nscoord* aPackingSpaceRemaining);
 
-  /**
-   * Returns true if aFrame is the frame for an element with
-   * "display:-webkit-box" or "display:-webkit-inline-box".
-   */
-  static bool IsLegacyBox(const nsIFrame* aFrame);
-
 protected:
   // Protected constructor & destructor
   explicit nsFlexContainerFrame(nsStyleContext* aContext)