--- a/devtools/server/actors/css-properties.js
+++ b/devtools/server/actors/css-properties.js
@@ -60,17 +60,17 @@ function generateCssProperties() {
 
     // Don't send colors over RDP, these will be re-attached by the front.
     let values = InspectorUtils.getCSSValuesForProperty(name);
     if (values.includes("aliceblue")) {
       values = values.filter(x => !colors.includes(x));
       values.unshift("COLOR");
     }
 
-    let subproperties = DOMUtils.getSubpropertiesForCSSProperty(name);
+    let subproperties = InspectorUtils.getSubpropertiesForCSSProperty(name);
 
     properties[name] = {
       isInherited: InspectorUtils.isInheritedProperty(name),
       values,
       supports,
       subproperties,
     };
   });

--- a/dom/webidl/InspectorUtils.webidl
+++ b/dom/webidl/InspectorUtils.webidl
@@ -30,16 +30,17 @@ namespace InspectorUtils {
       unsigned long selectorIndex,
       [TreatNullAs=EmptyString] optional DOMString pseudo = "");
   boolean isInheritedProperty(DOMString property);
   sequence<DOMString> getCSSPropertyNames(optional PropertyNamesOptions options);
   [Throws] sequence<DOMString> getCSSValuesForProperty(DOMString property);
   [Throws] DOMString rgbToColorName(octet r, octet g, octet b);
   InspectorRGBATuple? colorToRGBA(DOMString colorString);
   boolean isValidCSSColor(DOMString colorString);
+  [Throws] sequence<DOMString> getSubpropertiesForCSSProperty(DOMString property);
 };
 
 dictionary PropertyNamesOptions {
   boolean includeAliases = false;
 };
 
 dictionary InspectorRGBATuple {
   /*

--- a/layout/inspector/InspectorUtils.h
+++ b/layout/inspector/InspectorUtils.h
@@ -129,16 +129,26 @@ public:
   static void ColorToRGBA(GlobalObject& aGlobal,
                           const nsAString& aColorString,
                           Nullable<InspectorRGBATuple>& aResult);
 
   // Check whether a given color is a valid CSS color.
   static bool IsValidCSSColor(GlobalObject& aGlobal,
                               const nsAString& aColorString);
 
+  // Utilities for obtaining information about a CSS property.
+
+  // Get a list of the longhands corresponding to the given CSS property.  If
+  // the property is a longhand already, just returns the property itself.
+  // Throws on unsupported property names.
+  static void GetSubpropertiesForCSSProperty(GlobalObject& aGlobal,
+                                             const nsAString& aProperty,
+                                             nsTArray<nsString>& aResult,
+                                             ErrorResult& aRv);
+
 private:
   static already_AddRefed<nsStyleContext>
     GetCleanStyleContextForElement(Element* aElement, nsAtom* aPseudo);
 };
 
 } // namespace dom
 } // namespace mozilla
 

--- a/layout/inspector/inDOMUtils.cpp
+++ b/layout/inspector/inDOMUtils.cpp
@@ -547,59 +547,53 @@ static void GetOtherValuesForProperty(co
     InsertNoDuplicates(aArray, NS_LITERAL_STRING("repeating-radial-gradient"));
     InsertNoDuplicates(aArray, NS_LITERAL_STRING("-moz-linear-gradient"));
     InsertNoDuplicates(aArray, NS_LITERAL_STRING("-moz-radial-gradient"));
     InsertNoDuplicates(aArray, NS_LITERAL_STRING("-moz-repeating-linear-gradient"));
     InsertNoDuplicates(aArray, NS_LITERAL_STRING("-moz-repeating-radial-gradient"));
   }
 }
 
-NS_IMETHODIMP
-inDOMUtils::GetSubpropertiesForCSSProperty(const nsAString& aProperty,
-                                           uint32_t* aLength,
-                                           char16_t*** aValues)
+namespace mozilla {
+namespace dom {
+
+/* static */ void
+InspectorUtils::GetSubpropertiesForCSSProperty(GlobalObject& aGlobal,
+                                               const nsAString& aProperty,
+                                               nsTArray<nsString>& aResult,
+                                               ErrorResult& aRv)
 {
   nsCSSPropertyID propertyID =
     nsCSSProps::LookupProperty(aProperty, CSSEnabledState::eForAllContent);
 
   if (propertyID == eCSSProperty_UNKNOWN) {
-    return NS_ERROR_FAILURE;
+    aRv.Throw(NS_ERROR_FAILURE);
+    return;
   }
 
   if (propertyID == eCSSPropertyExtra_variable) {
-    *aValues = static_cast<char16_t**>(moz_xmalloc(sizeof(char16_t*)));
-    (*aValues)[0] = ToNewUnicode(aProperty);
-    *aLength = 1;
-    return NS_OK;
+    aResult.AppendElement(aProperty);
+    return;
   }
 
   if (!nsCSSProps::IsShorthand(propertyID)) {
-    *aValues = static_cast<char16_t**>(moz_xmalloc(sizeof(char16_t*)));
-    (*aValues)[0] = ToNewUnicode(nsCSSProps::GetStringValue(propertyID));
-    *aLength = 1;
-    return NS_OK;
+    nsString* name = aResult.AppendElement();
+    CopyASCIItoUTF16(nsCSSProps::GetStringValue(propertyID), *name);
+    return;
   }
 
-  // Count up how many subproperties we have.
-  size_t subpropCount = 0;
-  for (const nsCSSPropertyID *props = nsCSSProps::SubpropertyEntryFor(propertyID);
+  for (const nsCSSPropertyID* props = nsCSSProps::SubpropertyEntryFor(propertyID);
        *props != eCSSProperty_UNKNOWN; ++props) {
-    ++subpropCount;
+    nsString* name = aResult.AppendElement();
+    CopyASCIItoUTF16(nsCSSProps::GetStringValue(*props), *name);
   }
+}
 
-  *aValues =
-    static_cast<char16_t**>(moz_xmalloc(subpropCount * sizeof(char16_t*)));
-  *aLength = subpropCount;
-  for (const nsCSSPropertyID *props = nsCSSProps::SubpropertyEntryFor(propertyID),
-                           *props_start = props;
-       *props != eCSSProperty_UNKNOWN; ++props) {
-    (*aValues)[props-props_start] = ToNewUnicode(nsCSSProps::GetStringValue(*props));
-  }
-  return NS_OK;
-}
+} // namespace dom
+} // namespace mozilla
 
 NS_IMETHODIMP
 inDOMUtils::CssPropertyIsShorthand(const nsAString& aProperty, bool *_retval)
 {
   nsCSSPropertyID propertyID =
     nsCSSProps::LookupProperty(aProperty, CSSEnabledState::eForAllContent);
   if (propertyID == eCSSProperty_UNKNOWN) {
     return NS_ERROR_FAILURE;

--- a/layout/inspector/inIDOMUtils.idl
+++ b/layout/inspector/inIDOMUtils.idl
@@ -15,24 +15,16 @@ interface nsIDOMNode;
 interface nsIDOMNodeList;
 interface nsIDOMFontFaceList;
 interface nsIDOMRange;
 interface nsIDOMCSSStyleSheet;
 
 [scriptable, uuid(362e98c3-82c2-4ad8-8dcb-00e8e4eab497)]
 interface inIDOMUtils : nsISupports
 {
-  // Utilities for obtaining information about a CSS property.
-
-  // Get a list of the longhands corresponding to the given CSS property.  If
-  // the property is a longhand already, just returns the property itself.
-  // Throws on unsupported property names.
-  void getSubpropertiesForCSSProperty(in AString aProperty,
-                                      [optional] out unsigned long aLength,
-                                      [array, size_is(aLength), retval] out wstring aValues);
   // Check whether a given CSS property is a shorthand.  Throws on unsupported
   // property names.
   bool cssPropertyIsShorthand(in AString aProperty);
 
   // Check whether values of the given type are valid values for the property.
   // For shorthands, checks whether there's a corresponding longhand property
   // that accepts values of this type.  Throws on unsupported properties or
   // unknown types.

--- a/layout/inspector/tests/test_bug1006595.html
+++ b/layout/inspector/tests/test_bug1006595.html
@@ -6,38 +6,41 @@ https://bugzilla.mozilla.org/show_bug.cg
 <head>
   <meta charset="utf-8">
   <title>Test for Bug 1006595</title>
   <script type="application/javascript" src="/tests/SimpleTest/SimpleTest.js"></script>
   <link rel="stylesheet" type="text/css" href="/tests/SimpleTest/test.css"/>
   <script type="application/javascript">
 
   /** Test for Bug 1006595 **/
+
+  const InspectorUtils = SpecialPowers.InspectorUtils;
+
   function arraysEqual(arr1, arr2, message) {
     is(arr1.length, arr2.length, message + " length");
     for (var i = 0; i < arr1.length; ++i) {
       is(arr1[i], arr2[i], message + " element at index " + i);
     }
   }
   var utils = SpecialPowers.Cc["@mozilla.org/inspector/dom-utils;1"]
     .getService(SpecialPowers.Ci.inIDOMUtils);
 
-  var paddingSubProps = utils.getSubpropertiesForCSSProperty("padding");
+  var paddingSubProps = InspectorUtils.getSubpropertiesForCSSProperty("padding");
   arraysEqual(paddingSubProps,
               [ "padding-top",
                 "padding-right",
                 "padding-bottom",
                 "padding-left" ],
               "'padding' subproperties");
 
-  var displaySubProps = utils.getSubpropertiesForCSSProperty("color");
+  var displaySubProps = InspectorUtils.getSubpropertiesForCSSProperty("color");
   arraysEqual(displaySubProps, [ "color" ],
               "'color' subproperties");
 
-  var varProps = utils.getSubpropertiesForCSSProperty("--foo");
+  var varProps = InspectorUtils.getSubpropertiesForCSSProperty("--foo");
   arraysEqual(varProps, ["--foo"], "'--foo' subproperties");
 
   ok(utils.cssPropertyIsShorthand("padding"), "'padding' is a shorthand")
   ok(!utils.cssPropertyIsShorthand("color"), "'color' is not a shorthand")
 
   ok(utils.cssPropertySupportsType("padding", utils.TYPE_LENGTH),
      "'padding' can be a length");
   ok(!utils.cssPropertySupportsType("padding", utils.TYPE_COLOR),