diff --git a/layout/style/nsStyleSet.cpp b/layout/style/nsStyleSet.cpp
index 11e7f18e22..a69bec00b5 100644
--- a/layout/style/nsStyleSet.cpp
+++ b/layout/style/nsStyleSet.cpp
@@ -530,7 +530,7 @@ nsStyleSet::GatherRuleProcessors(SheetType aType)
         scope->SetIsScopedStyleRoot();
 
         // Create a rule processor for the scope.
-        nsTArray<RefPtr<CSSStyleSheet>> sheetsForScope;
+        nsTArray<RefPtr<CSSStyleSheet>> sheetsForScope(end - start);
         sheetsForScope.AppendElements(sheets.Elements() + start, end - start);
         nsCSSRuleProcessor* oldRP = oldScopedRuleProcessorHash.Get(scope);
         mScopedDocSheetRuleProcessors.AppendElement
diff --git a/layout/svg/nsCSSFilterInstance.cpp b/layout/svg/nsCSSFilterInstance.cpp
index 79bf83b949..80388f77ce 100644
--- a/layout/svg/nsCSSFilterInstance.cpp
+++ b/layout/svg/nsCSSFilterInstance.cpp
@@ -409,7 +409,7 @@ nsCSSFilterInstance::SetBounds(FilterPrimitiveDescription& aDescr,
   nsIntRect inputBounds = (inputIndex < 0) ?
     mTargetBoundsInFilterSpace : aPrimitiveDescrs[inputIndex].PrimitiveSubregion();
 
-  nsTArray<nsIntRegion> inputExtents;
+  AutoTArray<nsIntRegion, 1> inputExtents;
   inputExtents.AppendElement(inputBounds);
 
   nsIntRegion outputExtents =
diff --git a/layout/svg/nsSVGEffects.cpp b/layout/svg/nsSVGEffects.cpp
index 0fb68ddd1b..8ca9ca13e8 100644
--- a/layout/svg/nsSVGEffects.cpp
+++ b/layout/svg/nsSVGEffects.cpp
@@ -670,6 +670,7 @@ nsSVGEffects::EffectProperties::GetMaskFrames()
 
   bool ok = false;
   const nsTArray<RefPtr<nsSVGPaintingProperty>>& props = mMask->GetProps();
+  result.SetCapacity(props.Length());
   for (size_t i = 0; i < props.Length(); i++) {
     nsSVGMaskFrame* maskFrame =
       static_cast<nsSVGMaskFrame *>(props[i]->GetReferencedFrame(
@@ -731,6 +732,7 @@ nsSVGRenderingObserverList::InvalidateAll()
     return;
 
   AutoTArray<nsSVGRenderingObserver*,10> observers;
+  observers.SetCapacity(mObservers.Count());
 
   for (auto it = mObservers.Iter(); !it.Done(); it.Next()) {
     observers.AppendElement(it.Get()->GetKey());
@@ -749,6 +751,7 @@ nsSVGRenderingObserverList::InvalidateAllForReflow()
     return;
 
   AutoTArray<nsSVGRenderingObserver*,10> observers;
+  observers.SetCapacity(mObservers.Count());
 
   for (auto it = mObservers.Iter(); !it.Done(); it.Next()) {
     nsSVGRenderingObserver* obs = it.Get()->GetKey();
@@ -767,6 +770,7 @@ void
 nsSVGRenderingObserverList::RemoveAll()
 {
   AutoTArray<nsSVGRenderingObserver*,10> observers;
+  observers.SetCapacity(mObservers.Count());
 
   for (auto it = mObservers.Iter(); !it.Done(); it.Next()) {
     observers.AppendElement(it.Get()->GetKey());