--- a/dom/storage/LocalStorageManager.cpp
+++ b/dom/storage/LocalStorageManager.cpp
@@ -137,17 +137,17 @@ LocalStorageManager::CreateOrigin(const 
   // Note: some hard-coded sqlite statements are dependent on the format this
   // method returns.  Changing this without updating those sqlite statements
   // will cause malfunction.
 
   nsAutoCString scope;
   scope.Append(aOriginSuffix);
   scope.Append(':');
   scope.Append(aOriginNoSuffix);
-  return scope;
+  return std::move(scope);
 }
 
 LocalStorageCache*
 LocalStorageManager::GetCache(const nsACString& aOriginSuffix,
                               const nsACString& aOriginNoSuffix)
 {
   CacheOriginHashtable* table = mCaches.LookupOrAdd(aOriginSuffix);
   LocalStorageCacheHashKey* entry = table->GetEntry(aOriginNoSuffix);

--- a/gfx/thebes/gfxPlatform.cpp
+++ b/gfx/thebes/gfxPlatform.cpp
@@ -1663,17 +1663,17 @@ gfxPlatform::GetDefaultFontName(const ns
     nsAutoString result;
 
     gfxFontFamily* fontFamily = gfxPlatformFontList::PlatformFontList()->
         GetDefaultFontFamily(aLangGroup, aGenericFamily);
     if (fontFamily) {
       fontFamily->LocalizedName(result);
     } // (else, leave 'result' empty)
 
-    return result;
+    return std::move(result);
 }
 
 bool
 gfxPlatform::DownloadableFontsEnabled()
 {
     if (mAllowDownloadableFonts == UNINITIALIZED_VALUE) {
         mAllowDownloadableFonts =
             Preferences::GetBool(GFX_DOWNLOADABLE_FONTS_ENABLED, false);

--- a/layout/painting/DisplayItemClip.cpp
+++ b/layout/painting/DisplayItemClip.cpp
@@ -540,17 +540,17 @@ DisplayItemClip::ToString() const
     for (uint32_t i = 0; i < mRoundedClipRects.Length(); ++i) {
       const RoundedRect& r = mRoundedClipRects[i];
       str.AppendPrintf(" [%d,%d,%d,%d corners %d,%d,%d,%d,%d,%d,%d,%d]",
                        r.mRect.x, r.mRect.y, r.mRect.width, r.mRect.height,
                        r.mRadii[0], r.mRadii[1], r.mRadii[2], r.mRadii[3],
                        r.mRadii[4], r.mRadii[5], r.mRadii[6], r.mRadii[7]);
     }
   }
-  return str;
+  return std::move(str);
 }
 
 void
 DisplayItemClip::ToComplexClipRegions(int32_t aAppUnitsPerDevPixel,
                                       const layers::StackingContextHelper& aSc,
                                       nsTArray<wr::ComplexClipRegion>& aOutArray) const
 {
   for (uint32_t i = 0; i < mRoundedClipRects.Length(); i++) {

--- a/layout/painting/DisplayItemClipChain.cpp
+++ b/layout/painting/DisplayItemClipChain.cpp
@@ -74,17 +74,17 @@ DisplayItemClipChain::ToString(const Dis
       str.AppendPrintf("0x%p <%s> [0x%p]", sc, sc->mClip.ToString().get(), sc->mASR->mScrollableFrame);
     } else {
       str.AppendPrintf("0x%p <%s> [root asr]", sc, sc->mClip.ToString().get());
     }
     if (sc->mParent) {
       str.AppendLiteral(", ");
     }
   }
-  return str;
+  return std::move(str);
 }
 
 bool
 DisplayItemClipChain::HasRoundedCorners() const
 {
   return mClip.GetRoundedRectCount() > 0 || (mParent && mParent->HasRoundedCorners());
 }
 

--- a/layout/painting/nsDisplayList.cpp
+++ b/layout/painting/nsDisplayList.cpp
@@ -169,17 +169,17 @@ ActiveScrolledRoot::ToString(const Activ
 {
   nsAutoCString str;
   for (auto* asr = aActiveScrolledRoot; asr; asr = asr->mParent) {
     str.AppendPrintf("<0x%p>", asr->mScrollableFrame);
     if (asr->mParent) {
       str.AppendLiteral(", ");
     }
   }
-  return str;
+  return std::move(str);
 }
 
 static inline CSSAngle
 MakeCSSAngle(const nsCSSValue& aValue)
 {
   return CSSAngle(aValue.GetAngleValue(), aValue.GetUnit());
 }
 

--- a/toolkit/components/extensions/WebExtensionPolicy.cpp
+++ b/toolkit/components/extensions/WebExtensionPolicy.cpp
@@ -454,30 +454,30 @@ WebExtensionPolicy::IsRestrictedURI(cons
 
 nsCString
 WebExtensionPolicy::BackgroundPageHTML() const
 {
   nsAutoCString result;
 
   if (mBackgroundScripts.IsNull()) {
     result.SetIsVoid(true);
-    return result;
+    return std::move(result);
   }
 
   result.AppendLiteral(kBackgroundPageHTMLStart);
 
   for (auto& script : mBackgroundScripts.Value()) {
     nsCString escaped;
     nsAppendEscapedHTML(NS_ConvertUTF16toUTF8(script), escaped);
 
     result.AppendPrintf(kBackgroundPageHTMLScript, escaped.get());
   }
 
   result.AppendLiteral(kBackgroundPageHTMLEnd);
-  return result;
+  return std::move(result);
 }
 
 void
 WebExtensionPolicy::Localize(const nsAString& aInput, nsString& aOutput) const
 {
   mLocalizeCallback->Call(aInput, aOutput);
 }
 

--- a/toolkit/components/url-classifier/LookupCache.h
+++ b/toolkit/components/url-classifier/LookupCache.h
@@ -55,17 +55,17 @@ public:
     }
   }
 
   nsAutoCString PartialHashHex() const {
     nsAutoCString hex;
     for (size_t i = 0; i < mPartialHashLength; i++) {
       hex.AppendPrintf("%.2X", hash.complete.buf[i]);
     }
-    return hex;
+    return std::move(hex);
   }
 
   bool Confirmed() const { return mConfirmed || mProtocolConfirmed; }
 
   // True if we have a complete match for this hash in the table.
   bool Complete() const { return mPartialHashLength == COMPLETE_SIZE; }
 
   // True if this is a noise entry, i.e. an extra entry