diff --git a/dom/smil/nsSMILKeySpline.h b/dom/smil/nsSMILKeySpline.h
index 36c14fec1fef..c651c278fa5c 100644
--- a/dom/smil/nsSMILKeySpline.h
+++ b/dom/smil/nsSMILKeySpline.h
@@ -16,7 +16,14 @@
 class nsSMILKeySpline
 {
 public:
-  nsSMILKeySpline() { /* caller must call Init later */ }
+  nsSMILKeySpline()
+    : mX1(0)
+    , mY1(0)
+    , mX2(0)
+    , mY2(0)
+  { 
+    /* caller must call Init later */\
+  }
 
   /**
    * Creates a new key spline control point description.
@@ -26,6 +33,10 @@ public:
    */
   nsSMILKeySpline(double aX1, double aY1,
                   double aX2, double aY2)
+    : mX1(0)
+    , mY1(0)
+    , mX2(0)
+    , mY2(0)
   {
     Init(aX1, aY1, aX2, aY2);
   }
diff --git a/dom/smil/nsSMILValue.cpp b/dom/smil/nsSMILValue.cpp
index e5f904bb591e..fe67af84f2f9 100644
--- a/dom/smil/nsSMILValue.cpp
+++ b/dom/smil/nsSMILValue.cpp
@@ -14,6 +14,7 @@
 nsSMILValue::nsSMILValue(const nsISMILType* aType)
   : mType(nsSMILNullType::Singleton())
 {
+  mU.mBool = false;
   if (!aType) {
     NS_ERROR("Trying to construct nsSMILValue with null mType pointer");
     return;
diff --git a/dom/svg/DOMSVGPathSegList.h b/dom/svg/DOMSVGPathSegList.h
index 5683f5f6dd33..84bb5ef036fb 100644
--- a/dom/svg/DOMSVGPathSegList.h
+++ b/dom/svg/DOMSVGPathSegList.h
@@ -230,7 +230,10 @@ private:
    * want to create the DOMSVGPathSeg items lazily on demand.
    */
   struct ItemProxy {
-    ItemProxy(){}
+    ItemProxy()
+      : mItem(nullptr)
+      , mInternalDataIndex(0)
+    {}
     ItemProxy(DOMSVGPathSeg *aItem, uint32_t aInternalDataIndex)
       : mItem(aItem)
       , mInternalDataIndex(aInternalDataIndex)
diff --git a/dom/svg/SVGAnimatedNumberList.h b/dom/svg/SVGAnimatedNumberList.h
index 6b5e78c79c81..7300b286e51a 100644
--- a/dom/svg/SVGAnimatedNumberList.h
+++ b/dom/svg/SVGAnimatedNumberList.h
@@ -43,7 +43,9 @@ class SVGAnimatedNumberList
   friend class DOMSVGNumberList;
 
 public:
-  SVGAnimatedNumberList() {}
+  SVGAnimatedNumberList()
+    : mIsBaseSet(false)
+  {}
 
   /**
    * Because it's so important that mBaseVal and its DOMSVGNumberList wrapper
diff --git a/dom/svg/SVGGeometryElement.h b/dom/svg/SVGGeometryElement.h
index cef22975dade..54a7ea61412e 100644
--- a/dom/svg/SVGGeometryElement.h
+++ b/dom/svg/SVGGeometryElement.h
@@ -112,7 +112,11 @@ public:
   {
   public:
     SimplePath()
-      : mType(NONE)
+      : mX(0.0)
+      , mY(0.0)
+      , mWidthOrX2(0.0)
+      , mHeightOrY2(0.0)
+      , mType(NONE)
     {}
     bool IsPath() const {
       return mType != NONE;
diff --git a/dom/svg/SVGLength.h b/dom/svg/SVGLength.h
index 12757aa4fb58..6f4847be184e 100644
--- a/dom/svg/SVGLength.h
+++ b/dom/svg/SVGLength.h
@@ -33,10 +33,8 @@ class SVGLength
 public:
 
   SVGLength()
-#ifdef DEBUG
     : mValue(0.0f)
     , mUnit(dom::SVGLengthBinding::SVG_LENGTHTYPE_UNKNOWN) // caught by IsValid()
-#endif
   {}
 
   SVGLength(float aValue, uint8_t aUnit)
diff --git a/dom/svg/SVGLengthList.h b/dom/svg/SVGLengthList.h
index f9c8bd35613f..f74fb5ba6cf3 100644
--- a/dom/svg/SVGLengthList.h
+++ b/dom/svg/SVGLengthList.h
@@ -311,6 +311,8 @@ public:
 
   SVGUserUnitList()
     : mList(nullptr)
+    , mElement(nullptr)
+    , mAxis(0)
   {}
 
   void Init(const SVGLengthList *aList, nsSVGElement *aElement, uint8_t aAxis) {
diff --git a/dom/svg/SVGMotionSMILType.cpp b/dom/svg/SVGMotionSMILType.cpp
index 4b29f882a72f..524d4672bd7e 100644
--- a/dom/svg/SVGMotionSMILType.cpp
+++ b/dom/svg/SVGMotionSMILType.cpp
@@ -64,7 +64,10 @@ struct MotionSegment
   // Default constructor just locks us into being a Translation, and leaves
   // other fields uninitialized (since client is presumably about to set them)
   MotionSegment()
-    : mSegmentType(eSegmentType_Translation)
+    : mRotateType(eRotateType_Auto),
+      mRotateAngle(0.0),
+      mSegmentType(eSegmentType_Translation),
+      mU{}
   { }
 
   // Constructor for a translation
diff --git a/dom/svg/nsSVGViewBox.h b/dom/svg/nsSVGViewBox.h
index edf511e983d3..a0725a92d305 100644
--- a/dom/svg/nsSVGViewBox.h
+++ b/dom/svg/nsSVGViewBox.h
@@ -32,7 +32,13 @@ struct nsSVGViewBoxRect
   float width, height;
   bool none;
 
-  nsSVGViewBoxRect() : none(true) {}
+  nsSVGViewBoxRect()
+    : x(0.0)
+    , y(0.0)
+    , width(0.0)
+    , height(0.0)
+    , none(true)
+  {}
   nsSVGViewBoxRect(float aX, float aY, float aWidth, float aHeight) :
     x(aX), y(aY), width(aWidth), height(aHeight), none(false) {}
   nsSVGViewBoxRect(const nsSVGViewBoxRect& rhs) :
diff --git a/layout/svg/SVGContextPaint.h b/layout/svg/SVGContextPaint.h
index 9beba95adb68..277d67df5537 100644
--- a/layout/svg/SVGContextPaint.h
+++ b/layout/svg/SVGContextPaint.h
@@ -190,7 +190,10 @@ public:
   float GetStrokeOpacity() const override { return mStrokeOpacity; }
 
   struct Paint {
-    Paint() : mPaintType(eStyleSVGPaintType_None) {}
+    Paint()
+      : mPaintDefinition{}
+      , mPaintType(eStyleSVGPaintType_None)
+    {}
 
     void SetPaintServer(nsIFrame* aFrame,
                         const gfxMatrix& aContextMatrix,
diff --git a/layout/svg/SVGTextFrame.cpp b/layout/svg/SVGTextFrame.cpp
index 8aaafcfae26c..f75fbc24867b 100644
--- a/layout/svg/SVGTextFrame.cpp
+++ b/layout/svg/SVGTextFrame.cpp
@@ -2439,10 +2439,12 @@ CharIterator::CharIterator(SVGTextFrame* aSVGTextFrame,
     mFrameForTrimCheck(nullptr),
     mTrimmedOffset(0),
     mTrimmedLength(0),
+    mTextRun(nullptr),
     mTextElementCharIndex(0),
     mGlyphStartTextElementCharIndex(0),
-    mLengthAdjustScaleFactor(aSVGTextFrame->mLengthAdjustScaleFactor)
-  , mPostReflow(aPostReflow)
+    mGlyphUndisplayedCharacters(0),
+    mLengthAdjustScaleFactor(aSVGTextFrame->mLengthAdjustScaleFactor),
+    mPostReflow(aPostReflow)
 {
   if (!AtEnd()) {
     mSkipCharsIterator = TextFrame()->EnsureTextRun(nsTextFrame::eInflated);
@@ -2772,7 +2774,8 @@ public:
       mContext(aContext),
       mFrame(aFrame),
       mCanvasTM(aCanvasTM),
-      mImgParams(aImgParams)
+      mImgParams(aImgParams),
+      mColor(0)
   {
   }