new file mode 100644
--- /dev/null
+++ b/mfbt/RollingMean.h
@@ -0,0 +1,109 @@
+/* -*- Mode: C++; tab-w idth: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+/* A set abstraction for enumeration values. */
+
+#ifndef mozilla_RollingMean_h_
+#define mozilla_RollingMean_h_
+
+#include "mozilla/Assertions.h"
+#include "mozilla/TypeTraits.h"
+#include "mozilla/Vector.h"
+
+#include <algorithm>
+#include <stddef.h>
+#include <stdint.h>
+
+namespace mozilla {
+
+/**
+ * RollingMean<T> calculates a rolling mean of the values it is given. It
+ * accumulates the total as values are added and removed. The second type
+ * argument S specifies the type of the total. This may need to be a bigger
+ * type in order to maintain that the sum of all values in the average doesn't
+ * exceed the maximum input value.
+ *
+ * WARNING: Float types are not supported due to rounding errors.
+ */
+template<typename T, typename S>
+class RollingMean
+{
+  private:
+    size_t mInsertIndex;
+    size_t mMaxValues;
+    Vector<T> mValues;
+    S mTotal;
+
+  public:
+    static_assert(!IsFloatingPoint<T>::value,
+                  "floating-point types are unsupported due to rounding "
+                  "errors");
+
+    RollingMean(size_t aMaxValues)
+      : mInsertIndex(0),
+        mMaxValues(aMaxValues),
+        mTotal(0)
+    {
+      MOZ_ASSERT(aMaxValues > 0);
+    }
+
+    RollingMean& operator=(RollingMean&& aOther) {
+      MOZ_ASSERT(this != &aOther, "self-assignment is forbidden");
+      this->~RollingMean();
+      new(this) RollingMean(aOther.mMaxValues);
+      mInsertIndex = aOther.mInsertIndex;
+      mTotal = aOther.mTotal;
+      mValues.swap(aOther.mValues);
+      return *this;
+    }
+
+    /**
+     * Insert a value into the rolling mean.
+     */
+    bool insert(T aValue) {
+      MOZ_ASSERT(mValues.length() <= mMaxValues);
+
+      if (mValues.length() == mMaxValues) {
+        mTotal = mTotal - mValues[mInsertIndex] + aValue;
+        mValues[mInsertIndex] = aValue;
+      } else {
+        if (!mValues.append(aValue))
+          return false;
+        mTotal = mTotal + aValue;
+      }
+
+      mInsertIndex = (mInsertIndex + 1) % mMaxValues;
+      return true;
+    }
+
+    /**
+     * Calculate the rolling mean.
+     */
+    T mean() {
+      MOZ_ASSERT(!empty());
+      return T(mTotal / mValues.length());
+    }
+
+    bool empty() {
+      return mValues.empty();
+    }
+
+    /**
+     * Remove all values from the rolling mean.
+     */
+    void clear() {
+      mValues.clear();
+      mInsertIndex = 0;
+      mTotal = T(0);
+    }
+
+    size_t maxValues() {
+      return mMaxValues;
+    }
+};
+
+} // namespace mozilla
+
+#endif // mozilla_RollingMean_h_

--- a/mfbt/Vector.h
+++ b/mfbt/Vector.h
@@ -964,17 +964,17 @@ inline void
 VectorBase<T, N, AP, TV>::erase(T* it)
 {
   MOZ_ASSERT(begin() <= it);
   MOZ_ASSERT(it < end());
   while (it + 1 < end()) {
     *it = *(it + 1);
     ++it;
   }
-  popBack();
+    popBack();
 }
 
 template<typename T, size_t N, class AP, class TV>
 template<typename U>
 MOZ_ALWAYS_INLINE bool
 VectorBase<T, N, AP, TV>::append(const U* insBegin, const U* insEnd)
 {
   MOZ_REENTRANCY_GUARD_ET_AL;

--- a/mfbt/common.mozbuild
+++ b/mfbt/common.mozbuild
@@ -43,16 +43,17 @@ mfbt_headers = [
     'NullPtr.h',
     'NumericLimits.h',
     'PodOperations.h',
     'Poison.h',
     'Range.h',
     'RangedPtr.h',
     'ReentrancyGuard.h',
     'RefPtr.h',
+    'RollingMean.h',
     'Scoped.h',
     'SHA1.h',
     'SplayTree.h',
     'TemplateLib.h',
     'ThreadLocal.h',
     'TypedEnum.h',
     'Types.h',
     'TypeTraits.h',

new file mode 100644
--- /dev/null
+++ b/mfbt/tests/TestRollingMean.cpp
@@ -0,0 +1,121 @@
+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this file,
+ * You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "mozilla/Assertions.h"
+#include "mozilla/RollingMean.h"
+
+using mozilla::RollingMean;
+
+class MyClass
+{
+  public:
+    uint32_t value;
+
+    MyClass(uint32_t val = 0) : value(val) {
+    }
+
+    bool operator==(const MyClass& other) const {
+      return value == other.value;
+    }
+
+    MyClass operator+(const MyClass& other) const {
+      return MyClass(value + other.value);
+    }
+
+    MyClass operator-(const MyClass& other) const {
+      return MyClass(value - other.value);
+    }
+
+    MyClass operator/(uint32_t div) const {
+      return MyClass(value / div);
+    }
+};
+
+class RollingMeanSuite
+{
+  public:
+    RollingMeanSuite()
+    { }
+
+    void runTests() {
+      testZero();
+      testClear();
+      testRolling();
+      testClass();
+      testMove();
+    }
+
+  private:
+    void testZero() {
+      RollingMean<uint32_t, uint64_t> mean(3);
+      MOZ_ASSERT(mean.empty());
+    }
+
+    void testClear() {
+      RollingMean<uint32_t, uint64_t> mean(3);
+
+      mean.insert(4);
+      MOZ_ASSERT(mean.mean() == 4);
+
+      mean.clear();
+      MOZ_ASSERT(mean.empty());
+
+      mean.insert(3);
+      MOZ_ASSERT(mean.mean() == 3);
+    }
+
+    void testRolling() {
+      RollingMean<uint32_t, uint64_t> mean(3);
+
+      mean.insert(10);
+      MOZ_ASSERT(mean.mean() == 10);
+
+      mean.insert(20);
+      MOZ_ASSERT(mean.mean() == 15);
+
+      mean.insert(35);
+      MOZ_ASSERT(mean.mean() == 21);
+
+      mean.insert(5);
+      MOZ_ASSERT(mean.mean() == 20);
+
+      mean.insert(10);
+      MOZ_ASSERT(mean.mean() == 16);
+    }
+
+    void testClass() {
+      RollingMean<MyClass, MyClass> mean(3);
+
+      mean.insert(MyClass(4));
+      MOZ_ASSERT(mean.mean() == MyClass(4));
+
+      mean.clear();
+      MOZ_ASSERT(mean.empty());
+    }
+
+    void testMove() {
+      RollingMean<uint32_t, uint64_t> mean(3);
+      mean = RollingMean<uint32_t, uint64_t>(4);
+      MOZ_ASSERT(mean.maxValues() == 4);
+
+      mean.insert(10);
+      MOZ_ASSERT(mean.mean() == 10);
+
+      mean = RollingMean<uint32_t, uint64_t>(3);
+      mean.insert(30);
+      mean.insert(40);
+      mean.insert(50);
+      mean.insert(60);
+      MOZ_ASSERT(mean.mean() == 50);
+    }
+
+};
+
+int main()
+{
+  RollingMeanSuite suite;
+  suite.runTests();
+  return 0;
+}

--- a/mfbt/tests/moz.build
+++ b/mfbt/tests/moz.build
@@ -10,16 +10,17 @@ CPP_UNIT_TESTS += [
     'TestCasting.cpp',
     'TestCeilingFloor.cpp',
     'TestCheckedInt.cpp',
     'TestCountZeroes.cpp',
     'TestEndian.cpp',
     'TestEnumSet.cpp',
     'TestFloatingPoint.cpp',
     'TestIntegerPrintfMacros.cpp',
+    'TestRollingMean.cpp',
     'TestSHA1.cpp',
     'TestTypedEnum.cpp',
     'TestTypeTraits.cpp',
     'TestWeakPtr.cpp',
 ]
 
 if not CONFIG['MOZ_ASAN']:
     CPP_UNIT_TESTS += [