diff --git a/layout/base/nsLayoutUtils.cpp b/layout/base/nsLayoutUtils.cpp
index ac7dacf8ac61..c7e4173f02fe 100644
--- a/layout/base/nsLayoutUtils.cpp
+++ b/layout/base/nsLayoutUtils.cpp
@@ -60,6 +60,7 @@
 #include "imgIRequest.h"
 #include "nsIImageLoadingContent.h"
 #include "nsCOMPtr.h"
+#include "nsCSSProps.h"
 #include "nsListControlFrame.h"
 #include "ImageLayers.h"
 #include "mozilla/arm.h"
@@ -92,6 +93,8 @@ using namespace mozilla::layers;
 using namespace mozilla::dom;
 using namespace mozilla::layout;
 
+#define FLEXBOX_ENABLED_PREF_NAME "layout.css.flexbox.enabled"
+
 #ifdef DEBUG
 // TODO: remove, see bug 598468.
 bool nsLayoutUtils::gPreventAssertInCompareTreePosition = false;
@@ -107,6 +110,13 @@ typedef FrameMetrics::ViewID ViewID;
 
 static ViewID sScrollIdCounter = FrameMetrics::START_SCROLL_ID;
 
+// These are indices into kDisplayKTable.  They'll be initialized
+// the first time that FlexboxEnabledPrefChangeCallback() is invoked.
+static int32_t sIndexOfFlexInDisplayTable;
+static int32_t sIndexOfInlineFlexInDisplayTable;
+// This tracks whether those ^^ indices have been initialized
+static bool sAreFlexKeywordIndicesInitialized = false;
+
 typedef nsDataHashtable<nsUint64HashKey, nsIContent*> ContentMap;
 static ContentMap* sContentMap = nullptr;
 static ContentMap& GetContentMap() {
@@ -117,6 +127,49 @@ static ContentMap& GetContentMap() {
   return *sContentMap;
 }
 
+// When the pref "layout.css.flexbox.enabled" changes, this function is invoked
+// to let us update kDisplayKTable, to selectively disable or restore the
+// entries for "-moz-flex" and "-moz-inline-flex" in that table.
+#ifdef MOZ_FLEXBOX
+static int
+FlexboxEnabledPrefChangeCallback(const char* aPrefName, void* aClosure)
+{
+  MOZ_ASSERT(strncmp(aPrefName, FLEXBOX_ENABLED_PREF_NAME,
+                     NS_ARRAY_LENGTH(FLEXBOX_ENABLED_PREF_NAME)) == 0,
+             "We only registered this callback for a single pref, so it "
+             "should only be called for that pref");
+
+  bool isFlexboxEnabled =
+    Preferences::GetBool(FLEXBOX_ENABLED_PREF_NAME, false);
+
+  if (!sAreFlexKeywordIndicesInitialized) {
+    // First run: find the position of "-moz-flex" and "-moz-inline-flex" in
+    // kDisplayKTable.
+    sIndexOfFlexInDisplayTable =
+      nsCSSProps::FindIndexOfKeyword(eCSSKeyword__moz_flex,
+                                     nsCSSProps::kDisplayKTable);
+    sIndexOfInlineFlexInDisplayTable =
+      nsCSSProps::FindIndexOfKeyword(eCSSKeyword__moz_inline_flex,
+                                     nsCSSProps::kDisplayKTable);
+
+    sAreFlexKeywordIndicesInitialized = true;
+  }
+
+  // OK -- now, stomp on or restore the "flex" entries in kDisplayKTable,
+  // depending on whether the flexbox pref is enabled vs. disabled.
+  if (sIndexOfFlexInDisplayTable >= 0) {
+    nsCSSProps::kDisplayKTable[sIndexOfFlexInDisplayTable] =
+      isFlexboxEnabled ? eCSSKeyword__moz_flex : eCSSKeyword_UNKNOWN;
+  }
+  if (sIndexOfInlineFlexInDisplayTable >= 0) {
+    nsCSSProps::kDisplayKTable[sIndexOfInlineFlexInDisplayTable] =
+      isFlexboxEnabled ? eCSSKeyword__moz_inline_flex : eCSSKeyword_UNKNOWN;
+  }
+
+  return 0;
+}
+#endif // MOZ_FLEXBOX
+
 bool
 nsLayoutUtils::HasAnimationsForCompositor(nsIContent* aContent,
                                           nsCSSProperty aProperty)
@@ -4580,6 +4633,12 @@ nsLayoutUtils::Initialize()
                                "font.size.inflation.lineThreshold");
   Preferences::AddIntVarCache(&sFontSizeInflationMappingIntercept,
                               "font.size.inflation.mappingIntercept");
+
+#ifdef MOZ_FLEXBOX
+  Preferences::RegisterCallback(FlexboxEnabledPrefChangeCallback,
+                                FLEXBOX_ENABLED_PREF_NAME);
+  FlexboxEnabledPrefChangeCallback(FLEXBOX_ENABLED_PREF_NAME, nullptr);
+#endif // MOZ_FLEXBOX
 }
 
 /* static */
@@ -4590,6 +4649,11 @@ nsLayoutUtils::Shutdown()
     delete sContentMap;
     sContentMap = nullptr;
   }
+
+#ifdef MOZ_FLEXBOX
+  Preferences::UnregisterCallback(FlexboxEnabledPrefChangeCallback,
+                                  FLEXBOX_ENABLED_PREF_NAME);
+#endif // MOZ_FLEXBOX
 }
 
 /* static */
diff --git a/layout/style/nsCSSProps.cpp b/layout/style/nsCSSProps.cpp
index 94878f3601d9..2e73b42bcc75 100644
--- a/layout/style/nsCSSProps.cpp
+++ b/layout/style/nsCSSProps.cpp
@@ -920,7 +920,7 @@ const int32_t nsCSSProps::kDirectionKTable[] = {
   eCSSKeyword_UNKNOWN,-1
 };
 
-const int32_t nsCSSProps::kDisplayKTable[] = {
+int32_t nsCSSProps::kDisplayKTable[] = {
   eCSSKeyword_none,               NS_STYLE_DISPLAY_NONE,
   eCSSKeyword_inline,             NS_STYLE_DISPLAY_INLINE,
   eCSSKeyword_block,              NS_STYLE_DISPLAY_BLOCK,
@@ -952,6 +952,9 @@ const int32_t nsCSSProps::kDisplayKTable[] = {
   eCSSKeyword__moz_groupbox,      NS_STYLE_DISPLAY_GROUPBOX,
 #endif
 #ifdef MOZ_FLEXBOX
+  // XXXdholbert NOTE: These currently need to be the last entries in the
+  // table, because the "is flexbox enabled" pref that disables these will
+  // disable all the entries after them, too.
   eCSSKeyword__moz_flex,          NS_STYLE_DISPLAY_FLEX,
   eCSSKeyword__moz_inline_flex,   NS_STYLE_DISPLAY_INLINE_FLEX,
 #endif // MOZ_FLEXBOX
@@ -1639,17 +1642,28 @@ const int32_t nsCSSProps::kColumnFillKTable[] = {
   eCSSKeyword_UNKNOWN, -1
 };
 
-bool
-nsCSSProps::FindKeyword(nsCSSKeyword aKeyword, const int32_t aTable[], int32_t& aResult)
+int32_t
+nsCSSProps::FindIndexOfKeyword(nsCSSKeyword aKeyword, const int32_t aTable[])
 {
   int32_t index = 0;
   while (eCSSKeyword_UNKNOWN != nsCSSKeyword(aTable[index])) {
     if (aKeyword == nsCSSKeyword(aTable[index])) {
-      aResult = aTable[index+1];
-      return true;
+      return index;
     }
     index += 2;
   }
+  return -1;
+}
+
+bool
+nsCSSProps::FindKeyword(nsCSSKeyword aKeyword, const int32_t aTable[],
+                        int32_t& aResult)
+{
+  int32_t index = FindIndexOfKeyword(aKeyword, aTable);
+  if (index >= 0) {
+    aResult = aTable[index + 1];
+    return true;
+  }
   return false;
 }
 
diff --git a/layout/style/nsCSSProps.h b/layout/style/nsCSSProps.h
index c13687c20ae9..f1d6f80d8c0d 100644
--- a/layout/style/nsCSSProps.h
+++ b/layout/style/nsCSSProps.h
@@ -194,6 +194,11 @@ public:
   // Sets the aStr param to the name of the propertyID
   static bool GetColorName(int32_t aPropID, nsCString &aStr);
 
+  // Returns the index of |aKeyword| in |aTable|, if it exists there;
+  // otherwise, returns -1.
+  // NOTE: Generally, clients should call FindKeyword() instead of this method.
+  static int32_t FindIndexOfKeyword(nsCSSKeyword aKeyword, const int32_t aTable[]);
+
   // Find |aKeyword| in |aTable|, if found set |aValue| to its corresponding value.
   // If not found, return false and do not set |aValue|.
   static bool FindKeyword(nsCSSKeyword aKeyword, const int32_t aTable[], int32_t& aValue);
@@ -373,7 +378,8 @@ public:
   static const int32_t kContentKTable[];
   static const int32_t kCursorKTable[];
   static const int32_t kDirectionKTable[];
-  static const int32_t kDisplayKTable[];
+  // Not const because we modify its entries when CSS prefs change.
+  static int32_t kDisplayKTable[];
   static const int32_t kElevationKTable[];
   static const int32_t kEmptyCellsKTable[];
 #ifdef MOZ_FLEXBOX