diff --git a/dom/animation/KeyframeEffectReadOnly.h b/dom/animation/KeyframeEffectReadOnly.h
index 848c569160d8..9591c9ffd570 100644
--- a/dom/animation/KeyframeEffectReadOnly.h
+++ b/dom/animation/KeyframeEffectReadOnly.h
@@ -48,8 +48,8 @@ class GlobalObject;
 class OwningElementOrCSSPseudoElement;
 class UnrestrictedDoubleOrKeyframeAnimationOptions;
 class UnrestrictedDoubleOrKeyframeEffectOptions;
-enum class IterationCompositeOperation : uint32_t;
-enum class CompositeOperation : uint32_t;
+enum class IterationCompositeOperation : uint8_t;
+enum class CompositeOperation : uint8_t;
 struct AnimationPropertyDetails;
 }
 
diff --git a/dom/base/nsGlobalWindow.h b/dom/base/nsGlobalWindow.h
index 0fcdfcc00054..91a69e04234b 100644
--- a/dom/base/nsGlobalWindow.h
+++ b/dom/base/nsGlobalWindow.h
@@ -115,7 +115,7 @@ class DocGroup;
 class External;
 class Function;
 class Gamepad;
-enum class ImageBitmapFormat : uint32_t;
+enum class ImageBitmapFormat : uint8_t;
 class IdleRequest;
 class IdleRequestCallback;
 class Location;
diff --git a/dom/base/nsIDocument.h b/dom/base/nsIDocument.h
index bac0c00e2c02..36e882f47d20 100644
--- a/dom/base/nsIDocument.h
+++ b/dom/base/nsIDocument.h
@@ -149,7 +149,7 @@ class MediaQueryList;
 class GlobalObject;
 class NodeFilter;
 class NodeIterator;
-enum class OrientationType : uint32_t;
+enum class OrientationType : uint8_t;
 class ProcessingInstruction;
 class Promise;
 class StyleSheetList;
@@ -3037,9 +3037,10 @@ protected:
 #ifdef MOZILLA_INTERNAL_API
   // Our visibility state
   mozilla::dom::VisibilityState mVisibilityState;
-  static_assert(sizeof(mozilla::dom::VisibilityState) == sizeof(uint32_t), "Error size of mVisibilityState and mDummy");
+  static_assert(sizeof(mozilla::dom::VisibilityState) == sizeof(uint8_t),
+                "Error size of mVisibilityState and mDummy");
 #else
-  uint32_t mDummy;
+  uint8_t mDummy;
 #endif
 
   // True if BIDI is enabled.
diff --git a/dom/bindings/Codegen.py b/dom/bindings/Codegen.py
index af171cad6bb3..4dde9ce0ffef 100644
--- a/dom/bindings/Codegen.py
+++ b/dom/bindings/Codegen.py
@@ -9625,15 +9625,25 @@ class CGEnum(CGThing):
     def nEnumStrings(self):
         return len(self.enum.values()) + 1
 
+    def underlyingType(self):
+        count = self.nEnumStrings()
+        if count <= 256:
+            return "uint8_t"
+        if count <= 65536:
+            return "uint16_t"
+        raise ValueError("Enum " + self.enum.identifier.name +
+                         " has more than 65536 values")
+
     def declare(self):
         decl = fill(
             """
-            enum class ${name} : uint32_t {
+            enum class ${name} : ${ty} {
               $*{enums}
               EndGuard_
             };
             """,
             name=self.enum.identifier.name,
+            ty=self.underlyingType(),
             enums=",\n".join(map(getEnumValueName, self.enum.values())) + ",\n")
         strings = CGNamespace(self.stringsNamespace(),
                               CGGeneric(declare="extern const EnumEntry %s[%d];\n"
diff --git a/dom/cache/CacheStorage.h b/dom/cache/CacheStorage.h
index 10310f6681ae..eac477524bf7 100644
--- a/dom/cache/CacheStorage.h
+++ b/dom/cache/CacheStorage.h
@@ -28,7 +28,7 @@ namespace ipc {
 
 namespace dom {
 
-enum class CacheStorageNamespace : uint32_t;
+enum class CacheStorageNamespace : uint8_t;
 class Promise;
 
 namespace workers {
diff --git a/dom/canvas/CanvasPath.h b/dom/canvas/CanvasPath.h
index e31b375cc3a1..1e4b6c308df7 100644
--- a/dom/canvas/CanvasPath.h
+++ b/dom/canvas/CanvasPath.h
@@ -15,7 +15,7 @@
 namespace mozilla {
 namespace dom {
 
-enum class CanvasWindingRule : uint32_t;
+enum class CanvasWindingRule : uint8_t;
 class SVGMatrix;
 
 class CanvasPath final :
diff --git a/dom/canvas/ImageBitmap.h b/dom/canvas/ImageBitmap.h
index 2119c6bda545..d362eb6ff4ca 100644
--- a/dom/canvas/ImageBitmap.h
+++ b/dom/canvas/ImageBitmap.h
@@ -52,7 +52,7 @@ class File;
 class HTMLCanvasElement;
 class HTMLImageElement;
 class HTMLVideoElement;
-enum class ImageBitmapFormat : uint32_t;
+enum class ImageBitmapFormat : uint8_t;
 class ImageData;
 class ImageUtils;
 template<typename T> class MapDataIntoBufferSource;
diff --git a/dom/canvas/ImageUtils.h b/dom/canvas/ImageUtils.h
index b64c33704ee1..185fe61db82e 100644
--- a/dom/canvas/ImageUtils.h
+++ b/dom/canvas/ImageUtils.h
@@ -21,7 +21,7 @@ class ErrorResult;
 namespace dom {
 
 struct ChannelPixelLayout;
-enum class ImageBitmapFormat : uint32_t;
+enum class ImageBitmapFormat : uint8_t;
 
 typedef nsTArray<ChannelPixelLayout> ImagePixelLayout;
 
diff --git a/dom/html/nsBrowserElement.h b/dom/html/nsBrowserElement.h
index d0e8a77c8a35..dfed45f2be72 100644
--- a/dom/html/nsBrowserElement.h
+++ b/dom/html/nsBrowserElement.h
@@ -22,8 +22,8 @@ struct BrowserElementDownloadOptions;
 struct BrowserElementExecuteScriptOptions;
 class BrowserElementNextPaintEventCallback;
 class DOMRequest;
-enum class BrowserFindCaseSensitivity: uint32_t;
-enum class BrowserFindDirection: uint32_t;
+enum class BrowserFindCaseSensitivity: uint8_t;
+enum class BrowserFindDirection: uint8_t;
 } // namespace dom
 
 class ErrorResult;
diff --git a/dom/media/webaudio/AudioContext.h b/dom/media/webaudio/AudioContext.h
index be9c59b95ccb..b74ca41e05e2 100644
--- a/dom/media/webaudio/AudioContext.h
+++ b/dom/media/webaudio/AudioContext.h
@@ -42,7 +42,7 @@ class AudioNodeStream;
 
 namespace dom {
 
-enum class AudioContextState : uint32_t;
+enum class AudioContextState : uint8_t;
 class AnalyserNode;
 class AudioBuffer;
 class AudioBufferSourceNode;
@@ -71,7 +71,7 @@ class WaveShaperNode;
 class PeriodicWave;
 struct PeriodicWaveConstraints;
 class Promise;
-enum class OscillatorType : uint32_t;
+enum class OscillatorType : uint8_t;
 
 // This is addrefed by the OscillatorNodeEngine on the main thread
 // and then used from the MSG thread.
diff --git a/dom/media/webaudio/AudioNodeStream.cpp b/dom/media/webaudio/AudioNodeStream.cpp
index 0e5aa3fc73d7..fafcfd2ce7bb 100644
--- a/dom/media/webaudio/AudioNodeStream.cpp
+++ b/dom/media/webaudio/AudioNodeStream.cpp
@@ -350,11 +350,6 @@ AudioNodeStream::SetChannelMixingParametersImpl(uint32_t aNumberOfChannels,
                                                 ChannelCountMode aChannelCountMode,
                                                 ChannelInterpretation aChannelInterpretation)
 {
-  // Make sure that we're not clobbering any significant bits by fitting these
-  // values in 16 bits.
-  MOZ_ASSERT(int(aChannelCountMode) < INT16_MAX);
-  MOZ_ASSERT(int(aChannelInterpretation) < INT16_MAX);
-
   mNumberOfInputChannels = aNumberOfChannels;
   mChannelCountMode = aChannelCountMode;
   mChannelInterpretation = aChannelInterpretation;
diff --git a/dom/workers/WorkerScope.h b/dom/workers/WorkerScope.h
index d473f4e27d32..38cf76ad6a06 100644
--- a/dom/workers/WorkerScope.h
+++ b/dom/workers/WorkerScope.h
@@ -23,7 +23,7 @@ class Console;
 class Crypto;
 class Function;
 class IDBFactory;
-enum class ImageBitmapFormat : uint32_t;
+enum class ImageBitmapFormat : uint8_t;
 class Performance;
 class Promise;
 class RequestOrUSVString;
diff --git a/gfx/vr/gfxVR.h b/gfx/vr/gfxVR.h
index 9b81b6bb14bc..558182f446dc 100644
--- a/gfx/vr/gfxVR.h
+++ b/gfx/vr/gfxVR.h
@@ -21,8 +21,8 @@ namespace layers {
 class PTextureParent;
 }
 namespace dom {
-enum class GamepadMappingType : uint32_t;
-enum class GamepadHand : uint32_t;
+enum class GamepadMappingType : uint8_t;
+enum class GamepadHand : uint8_t;
 struct GamepadPoseState;
 }
 namespace gfx {
diff --git a/layout/style/nsStyleConsts.h b/layout/style/nsStyleConsts.h
index 1835539fd956..4639f4ffe727 100644
--- a/layout/style/nsStyleConsts.h
+++ b/layout/style/nsStyleConsts.h
@@ -314,8 +314,8 @@ enum class StyleOrient : uint8_t {
 // See AnimationEffectReadOnly.webidl
 // and mozilla/dom/AnimationEffectReadOnlyBinding.h
 namespace dom {
-enum class PlaybackDirection : uint32_t;
-enum class FillMode : uint32_t;
+enum class PlaybackDirection : uint8_t;
+enum class FillMode : uint8_t;
 }
 
 // See nsStyleDisplay