diff --git a/dom/media/platforms/agnostic/bytestreams/H264.cpp b/dom/media/platforms/agnostic/bytestreams/H264.cpp
index a6e258876d4f..197ce2537b1b 100644
--- a/dom/media/platforms/agnostic/bytestreams/H264.cpp
+++ b/dom/media/platforms/agnostic/bytestreams/H264.cpp
@@ -984,6 +984,19 @@ uint32_t H264::ComputeMaxRefFrames(const mozilla::MediaByteBuffer* aExtraData) {
       if (DecodeRecoverySEI(decodedNAL, data)) {
         return FrameType::I_FRAME;
       }
+    } else if (nalType == H264_NAL_SLICE) {
+      // According to ITU-T Rec H.264 Table 7.3.3, read the slice type from
+      // slice_header, and the slice type 2 and 7 are representing I slice.
+      RefPtr<mozilla::MediaByteBuffer> decodedNAL = DecodeNALUnit(p, nalLen);
+      BitReader br(decodedNAL);
+      // Skip `first_mb_in_slice`
+      br.ReadUE();
+      // The value of slice type can go from 0 to 9, but the value between 5 to
+      // 9 are actually equal to 0 to 4.
+      const uint32_t sliceType = br.ReadUE() % 5;
+      if (sliceType == SLICE_TYPES::I_SLICE || sliceType == SI_SLICE) {
+        return FrameType::I_FRAME;
+      }
     }
   }
 
diff --git a/dom/media/platforms/agnostic/bytestreams/H264.h b/dom/media/platforms/agnostic/bytestreams/H264.h
index 6a771ce2dfbe..277714a61b61 100644
--- a/dom/media/platforms/agnostic/bytestreams/H264.h
+++ b/dom/media/platforms/agnostic/bytestreams/H264.h
@@ -36,6 +36,15 @@ enum NAL_TYPES {
   H264_NAL_SLICE_EXT_DVC = 21,
 };
 
+// According to ITU-T Rec H.264 (2017/04) Table 7.6.
+enum SLICE_TYPES {
+  P_SLICE = 0,
+  B_SLICE = 1,
+  I_SLICE = 2,
+  SP_SLICE = 3,
+  SI_SLICE = 4,
+};
+
 struct SPSData {
   bool operator==(const SPSData& aOther) const;
   bool operator!=(const SPSData& aOther) const;