new file mode 100644
--- /dev/null
+++ b/dom/media/gtest/TestOpusParser.cpp
@@ -0,0 +1,23 @@
+/* -*- 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 "gtest/gtest.h"
+#include "OpusParser.h"
+#include <algorithm>
+
+TEST(OpusParser, Mapping2)
+{
+  uint8_t validChannels[] = {
+    1,  3,  4,  6,   9,   11,  16,  18,  25,  27,  36,  38,  49,  51,  64,
+    66, 81, 83, 100, 102, 121, 123, 144, 146, 169, 171, 196, 198, 225, 227
+  };
+  for (uint8_t channels = 0; channels < 255; channels++) {
+    bool found = OpusParser::IsValidMapping2ChannelsCount(channels);
+    bool foundTable =
+      std::find(std::begin(validChannels), std::end(validChannels), channels) !=
+      std::end(validChannels);
+    EXPECT_EQ(found, foundTable);
+  }
+}

--- a/dom/media/gtest/moz.build
+++ b/dom/media/gtest/moz.build
@@ -28,16 +28,17 @@ UNIFIED_SOURCES += [
     'TestGMPRemoveAndDelete.cpp',
     'TestGMPUtils.cpp',
     'TestIntervalSet.cpp',
     'TestMediaDataDecoder.cpp',
     'TestMediaEventSource.cpp',
     'TestMediaMIMETypes.cpp',
     'TestMP3Demuxer.cpp',
     'TestMP4Demuxer.cpp',
+    'TestOpusParser.cpp',
     'TestRust.cpp',
     'TestVideoSegment.cpp',
     'TestVideoUtils.cpp',
     'TestVPXDecoding.cpp',
     'TestWebMBuffered.cpp',
 ]
 
 if CONFIG['MOZ_WEBM_ENCODER']:

--- a/dom/media/ogg/OpusParser.cpp
+++ b/dom/media/ogg/OpusParser.cpp
@@ -88,37 +88,19 @@ bool OpusParser::DecodeHeader(unsigned c
       if (mChannelMapping == 1 && mChannels > 8) {
         OPUS_LOG(LogLevel::Debug,
                  ("Invalid Opus file: too many channels (%d) for"
                   " mapping family 1.",
                   mChannels));
         return false;
       }
       if (mChannelMapping == 2) {
-        // https://tools.ietf.org/html/draft-ietf-codec-ambisonics-08#page-3
-        // For both channel mapping family 2 and family 3, the allowed numbers
-        // of channels: (1 + n)^2 + 2j for n = 0, 1, ..., 14 and j = 0 or 1,
-        // where n denotes the (highest) ambisonic order and j denotes whether
-        // or not there is a separate non-diegetic stereo stream Explicitly the
-        // allowed number of channels are 1, 3, 4, 6, 9, 11, 16, 18, 25, 27, 36,
-        // 38, 49, 51, 64, 66, 81, 83, 100, 102, 121, 123, 144, 146, 169, 171,
-        // 196, 198, 225, and 227.
-
-        // We use the property that int(sqrt(n)) == int(sqrt(n+2)) for n != 3
-        // which is handled by the test n^2 + 2 != channel
-        double val = sqrt(mChannels);
-        if (val == 0 || val > 15) {
+        if (!IsValidMapping2ChannelsCount(mChannels)) {
           return false;
         }
-        if (val != int32_t(val)) {
-          if (val * val + 2 != mChannels) {
-            // Not a valid channel count.
-            return false;
-          }
-        }
       }
       if (aLength > static_cast<unsigned>(20 + mChannels)) {
         mStreams = aData[19];
         mCoupledStreams = aData[20];
         int i;
         for (i = 0; i < mChannels; i++) {
           mMappingTable[i] = aData[21 + i];
         }
@@ -210,9 +192,31 @@ bool OpusParser::DecodeTags(unsigned cha
   OPUS_LOG(LogLevel::Debug, ("  vendor: %s", mVendorString.get()));
   for (uint32_t i = 0; i < mTags.Length(); i++) {
     OPUS_LOG(LogLevel::Debug, (" %s", mTags[i].get()));
   }
 #endif
   return true;
 }
 
+/* static */ bool
+OpusParser::IsValidMapping2ChannelsCount(uint8_t aChannels)
+{
+  // https://tools.ietf.org/html/draft-ietf-codec-ambisonics-08#page-4
+  // For both channel mapping family 2 and family 3, the allowed numbers
+  // of channels: (1 + n)^2 + 2j for n = 0, 1, ..., 14 and j = 0 or 1,
+  // where n denotes the (highest) ambisonic order and j denotes whether
+  // or not there is a separate non-diegetic stereo stream Explicitly the
+  // allowed number of channels are 1, 3, 4, 6, 9, 11, 16, 18, 25, 27, 36,
+  // 38, 49, 51, 64, 66, 81, 83, 100, 102, 121, 123, 144, 146, 169, 171,
+  // 196, 198, 225, and 227.
+
+  // We use the property that int(sqrt(n)) == int(sqrt(n+2)) for n != 3
+  // which is handled by the test n^2 + 2 != channel
+  if (aChannels < 1 || aChannels > 227) {
+    return false;
+  }
+  double val = sqrt(aChannels);
+  int32_t valInt = int32_t(val);
+  return val == valInt || valInt * valInt + 2 == aChannels;
+}
+
 } // namespace mozilla

--- a/dom/media/ogg/OpusParser.h
+++ b/dom/media/ogg/OpusParser.h
@@ -13,16 +13,17 @@ namespace mozilla {
 
 class OpusParser
 {
 public:
   OpusParser();
 
   bool DecodeHeader(unsigned char* aData, size_t aLength);
   bool DecodeTags(unsigned char* aData, size_t aLength);
+  static bool IsValidMapping2ChannelsCount(uint8_t aChannels);
 
   // Various fields from the Ogg Opus header.
   int mRate;        // Sample rate the decoder uses (always 48 kHz).
   uint32_t mNominalRate; // Original sample rate of the data (informational).
   int mChannels;    // Number of channels the stream encodes.
   uint16_t mPreSkip; // Number of samples to strip after decoder reset.
 #ifdef MOZ_SAMPLE_TYPE_FLOAT32
   float mGain;      // Gain to apply to decoder output.