diff --git a/xpcom/ds/nsISupportsArray.idl b/xpcom/ds/nsISupportsArray.idl
index cdf7fbc3fb56..75837d563b99 100644
--- a/xpcom/ds/nsISupportsArray.idl
+++ b/xpcom/ds/nsISupportsArray.idl
@@ -34,11 +34,9 @@ class nsISupportsArray;
 interface nsISupportsArray : nsICollection {
 
   [notxpcom] long IndexOf([const] in nsISupports aPossibleElement);
-  [notxpcom] long LastIndexOf([const] in nsISupports aPossibleElement);
   
   // xpcom-compatible versions
   long GetIndexOf(in nsISupports aPossibleElement);
-  long GetLastIndexOf(in nsISupports aPossibleElement);  
   
   [notxpcom] boolean InsertElementAt(in nsISupports aElement,
                                      in unsigned long aIndex);
diff --git a/xpcom/ds/nsSupportsArray.cpp b/xpcom/ds/nsSupportsArray.cpp
index 2a94f3e5bcbe..84eb1124ee3d 100644
--- a/xpcom/ds/nsSupportsArray.cpp
+++ b/xpcom/ds/nsSupportsArray.cpp
@@ -221,21 +221,6 @@ nsSupportsArray::IndexOf(const nsISupports* aPossibleElement)
   return -1;
 }
 
-NS_IMETHODIMP_(int32_t)
-nsSupportsArray::LastIndexOf(const nsISupports* aPossibleElement)
-{
-  if (0 < mCount) {
-    const nsISupports** start = (const nsISupports**)mArray;  // work around goofy compiler behavior
-    const nsISupports** ep = (start + mCount);
-    while (start <= --ep) {
-      if (aPossibleElement == *ep) {
-        return (ep - start);
-      }
-    }
-  }
-  return -1;
-}
-
 NS_IMETHODIMP_(bool)
 nsSupportsArray::InsertElementAt(nsISupports* aElement, uint32_t aIndex)
 {
diff --git a/xpcom/ds/nsSupportsArray.h b/xpcom/ds/nsSupportsArray.h
index e927a63689eb..f9959b12e578 100644
--- a/xpcom/ds/nsSupportsArray.h
+++ b/xpcom/ds/nsSupportsArray.h
@@ -61,7 +61,6 @@ public:
 
   // nsISupportsArray methods:
   NS_IMETHOD_(int32_t) IndexOf(const nsISupports* aPossibleElement) override;
-  NS_IMETHOD_(int32_t) LastIndexOf(const nsISupports* aPossibleElement) override;
 
   NS_IMETHOD GetIndexOf(nsISupports* aPossibleElement, int32_t* aResult) override
   {
@@ -69,12 +68,6 @@ public:
     return NS_OK;
   }
 
-  NS_IMETHOD GetLastIndexOf(nsISupports* aPossibleElement, int32_t* aResult) override
-  {
-    *aResult = LastIndexOf(aPossibleElement);
-    return NS_OK;
-  }
-
   MOZ_MUST_USE NS_IMETHOD_(bool)
   InsertElementAt(nsISupports* aElement, uint32_t aIndex) override;
 
diff --git a/xpcom/glue/tests/gtest/TestArray.cpp b/xpcom/glue/tests/gtest/TestArray.cpp
index 5a07bbfc05d2..18eb2a93f943 100644
--- a/xpcom/glue/tests/gtest/TestArray.cpp
+++ b/xpcom/glue/tests/gtest/TestArray.cpp
@@ -118,13 +118,10 @@ TEST(Array, main)
     CheckArray(array, 13, appendResult, 10);
 
 
-    // test IndexOf && LastIndexOf
+    // test IndexOf
     int32_t expectedIndex = 0;
     int32_t index = array->IndexOf(foo);
     EXPECT_EQ(index, expectedIndex);
-    expectedIndex = 12;
-    index = array->LastIndexOf(foo);
-    EXPECT_EQ(index, expectedIndex);
 
     // test ReplaceElementAt
     array->ReplaceElementAt(foo, 8);