diff --git a/image/RasterImage.cpp b/image/RasterImage.cpp
index 56e80412e35c..5edea6111358 100644
--- a/image/RasterImage.cpp
+++ b/image/RasterImage.cpp
@@ -1009,13 +1009,12 @@ RasterImage::Undefine(const char* prop) {
 }
 
 NS_IMETHODIMP
-RasterImage::GetKeys(uint32_t* count, char*** keys) {
+RasterImage::GetKeys(nsTArray<nsCString>& keys) {
   if (!mProperties) {
-    *count = 0;
-    *keys = nullptr;
+    keys.Clear();
     return NS_OK;
   }
-  return mProperties->GetKeys(count, keys);
+  return mProperties->GetKeys(keys);
 }
 
 void RasterImage::Discard() {
diff --git a/xpcom/ds/nsIProperties.idl b/xpcom/ds/nsIProperties.idl
index a87e14443846..cc80466bb524 100644
--- a/xpcom/ds/nsIProperties.idl
+++ b/xpcom/ds/nsIProperties.idl
@@ -42,5 +42,5 @@ interface nsIProperties : nsISupports
     /**
      *  Returns an array of the keys.
      */
-    void getKeys(out uint32_t count, [array, size_is(count), retval] out string keys);
+    Array<ACString> getKeys();
 };
diff --git a/xpcom/ds/nsPersistentProperties.cpp b/xpcom/ds/nsPersistentProperties.cpp
index 823182865169..cdb1a3cc55d3 100644
--- a/xpcom/ds/nsPersistentProperties.cpp
+++ b/xpcom/ds/nsPersistentProperties.cpp
@@ -543,7 +543,7 @@ nsPersistentProperties::Has(const char* aProp, bool* aResult) {
 }
 
 NS_IMETHODIMP
-nsPersistentProperties::GetKeys(uint32_t* aCount, char*** aKeys) {
+nsPersistentProperties::GetKeys(nsTArray<nsCString>& aKeys) {
   return NS_ERROR_NOT_IMPLEMENTED;
 }
 
diff --git a/xpcom/ds/nsProperties.cpp b/xpcom/ds/nsProperties.cpp
index 9322adae37f7..2a480c1e8a2b 100644
--- a/xpcom/ds/nsProperties.cpp
+++ b/xpcom/ds/nsProperties.cpp
@@ -52,32 +52,14 @@ nsProperties::Has(const char* prop, bool* result) {
 }
 
 NS_IMETHODIMP
-nsProperties::GetKeys(uint32_t* aCount, char*** aKeys) {
-  if (NS_WARN_IF(!aCount) || NS_WARN_IF(!aKeys)) {
-    return NS_ERROR_INVALID_ARG;
-  }
-
+nsProperties::GetKeys(nsTArray<nsCString>& aKeys) {
   uint32_t count = Count();
-  char** keys = (char**)moz_xmalloc(count * sizeof(char*));
-  uint32_t j = 0;
+  aKeys.SetCapacity(count);
 
   for (auto iter = this->Iter(); !iter.Done(); iter.Next()) {
-    const char* key = iter.Key();
-    keys[j] = strdup(key);
-
-    if (!keys[j]) {
-      // Free 'em all
-      for (uint32_t i = 0; i < j; i++) {
-        free(keys[i]);
-      }
-      free(keys);
-      return NS_ERROR_OUT_OF_MEMORY;
-    }
-    j++;
+    aKeys.AppendElement(iter.Key());
   }
 
-  *aCount = count;
-  *aKeys = keys;
   return NS_OK;
 }
 
diff --git a/xpcom/io/nsDirectoryService.cpp b/xpcom/io/nsDirectoryService.cpp
index 83cf6290d8b1..469c6611e706 100644
--- a/xpcom/io/nsDirectoryService.cpp
+++ b/xpcom/io/nsDirectoryService.cpp
@@ -122,7 +122,7 @@ nsDirectoryService::Undefine(const char* aProp) {
 }
 
 NS_IMETHODIMP
-nsDirectoryService::GetKeys(uint32_t* aCount, char*** aKeys) {
+nsDirectoryService::GetKeys(nsTArray<nsCString>& aKeys) {
   return NS_ERROR_NOT_IMPLEMENTED;
 }