--- a/xpcom/ds/nsISupportsArray.idl
+++ b/xpcom/ds/nsISupportsArray.idl
@@ -60,19 +60,16 @@ interface nsISupportsArray : nsICollecti
   
   void Compact();
   
 
   nsISupportsArray clone();
 
   [notxpcom] boolean RemoveElementsAt(in unsigned long aIndex,
                                       in unsigned long aCount);
-
-  [notxpcom] boolean SizeTo(in long aSize);
-  
 };
 
 %{C++
 
 // Construct and return a default implementation of nsISupportsArray:
 extern MOZ_MUST_USE nsresult
 NS_NewISupportsArray(nsISupportsArray** aInstancePtrResult);
 

--- a/xpcom/ds/nsSupportsArray.cpp
+++ b/xpcom/ds/nsSupportsArray.cpp
@@ -391,48 +391,16 @@ nsSupportsArray::Compact(void)
     }
 
     ::memcpy(mArray, oldArray, mCount * sizeof(nsISupports*));
     delete[] oldArray;
   }
   return NS_OK;
 }
 
-NS_IMETHODIMP_(bool)
-nsSupportsArray::SizeTo(int32_t aSize)
-{
-  NS_ASSERTION(aSize >= 0, "negative aSize!");
-
-  // XXX for aSize < mCount we could resize to mCount
-  if (mArraySize == (uint32_t)aSize || (uint32_t)aSize < mCount) {
-    return true;  // nothing to do
-  }
-
-  // switch back to autoarray if possible
-  nsISupports** oldArray = mArray;
-  if ((uint32_t)aSize <= kAutoArraySize) {
-    mArray = mAutoArray;
-    mArraySize = kAutoArraySize;
-  } else {
-    mArray = new nsISupports*[aSize];
-    if (!mArray) {
-      mArray = oldArray;
-      return false;
-    }
-    mArraySize = aSize;
-  }
-
-  ::memcpy(mArray, oldArray, mCount * sizeof(nsISupports*));
-  if (oldArray != mAutoArray) {
-    delete[] oldArray;
-  }
-
-  return true;
-}
-
 NS_IMETHODIMP
 nsSupportsArray::Enumerate(nsIEnumerator** aResult)
 {
   nsSupportsArrayEnumerator* e = new nsSupportsArrayEnumerator(this);
   *aResult = e;
   NS_ADDREF(e);
   return NS_OK;
 }

--- a/xpcom/ds/nsSupportsArray.h
+++ b/xpcom/ds/nsSupportsArray.h
@@ -112,18 +112,16 @@ public:
 
   NS_IMETHOD Compact(void) override;
 
   MOZ_MUST_USE NS_IMETHOD Clone(nsISupportsArray** aResult) override;
 
   MOZ_MUST_USE NS_IMETHOD_(bool)
   RemoveElementsAt(uint32_t aIndex, uint32_t aCount) override;
 
-  MOZ_MUST_USE NS_IMETHOD_(bool)
-  SizeTo(int32_t aSize) override;
 protected:
   void DeleteArray(void);
 
   bool GrowArrayBy(uint32_t aGrowBy);
 
   nsISupports** mArray;
   uint32_t mArraySize;
   uint32_t mCount;