diff --git a/xpcom/io/SlicedInputStream.cpp b/xpcom/io/SlicedInputStream.cpp
index afd00fe2da49..abb623137424 100644
--- a/xpcom/io/SlicedInputStream.cpp
+++ b/xpcom/io/SlicedInputStream.cpp
@@ -20,6 +20,8 @@ NS_INTERFACE_MAP_BEGIN(SlicedInputStream)
                                      mWeakCloneableInputStream || !mInputStream)
   NS_INTERFACE_MAP_ENTRY_CONDITIONAL(nsIIPCSerializableInputStream,
                                      mWeakIPCSerializableInputStream || !mInputStream)
+  NS_INTERFACE_MAP_ENTRY_CONDITIONAL(nsISeekableStream,
+                                     mWeakSeekableInputStream || !mInputStream)
   NS_INTERFACE_MAP_ENTRY_AMBIGUOUS(nsISupports, nsIInputStream)
 NS_INTERFACE_MAP_END
 
@@ -27,6 +29,7 @@ SlicedInputStream::SlicedInputStream(nsIInputStream* aInputStream,
                                      uint64_t aStart, uint64_t aLength)
   : mWeakCloneableInputStream(nullptr)
   , mWeakIPCSerializableInputStream(nullptr)
+  , mWeakSeekableInputStream(nullptr)
   , mStart(aStart)
   , mLength(aLength)
   , mCurPos(0)
@@ -39,6 +42,7 @@ SlicedInputStream::SlicedInputStream(nsIInputStream* aInputStream,
 SlicedInputStream::SlicedInputStream()
   : mWeakCloneableInputStream(nullptr)
   , mWeakIPCSerializableInputStream(nullptr)
+  , mWeakSeekableInputStream(nullptr)
   , mStart(0)
   , mLength(0)
   , mCurPos(0)
@@ -68,6 +72,12 @@ SlicedInputStream::SetSourceStream(nsIInputStream* aInputStream)
       SameCOMIdentity(aInputStream, serializableStream)) {
     mWeakIPCSerializableInputStream = serializableStream;
   }
+
+  nsCOMPtr<nsISeekableStream> seekableStream =
+    do_QueryInterface(aInputStream);
+  if (seekableStream && SameCOMIdentity(aInputStream, seekableStream)) {
+    mWeakSeekableInputStream = seekableStream;
+  }
 }
 
 NS_IMETHODIMP
@@ -325,3 +335,75 @@ SlicedInputStream::ExpectedSerializedLength()
 
   return mWeakIPCSerializableInputStream->ExpectedSerializedLength();
 }
+
+// nsISeekableStream
+
+NS_IMETHODIMP
+SlicedInputStream::Seek(int32_t aWhence, int64_t aOffset)
+{
+  NS_ENSURE_STATE(mInputStream);
+  NS_ENSURE_STATE(mWeakSeekableInputStream);
+
+  int64_t offset;
+  switch (aWhence) {
+    case NS_SEEK_SET:
+      offset = mStart + aOffset;
+      break;
+    case NS_SEEK_CUR:
+      offset = mStart + mCurPos + aOffset;
+      break;
+    case NS_SEEK_END:
+      offset = mStart + mLength + aOffset;
+      break;
+   default:
+     return NS_ERROR_ILLEGAL_VALUE;
+  }
+
+  if (offset < (int64_t)mStart || offset > (int64_t)(mStart + mLength)) {
+    return NS_ERROR_INVALID_ARG;
+  }
+
+  nsresult rv = mWeakSeekableInputStream->Seek(NS_SEEK_SET, offset);
+  if (NS_WARN_IF(NS_FAILED(rv))) {
+    return rv;
+  }
+
+  mCurPos = offset - mStart;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+SlicedInputStream::Tell(int64_t *aResult)
+{
+  NS_ENSURE_STATE(mInputStream);
+  NS_ENSURE_STATE(mWeakSeekableInputStream);
+
+  int64_t tell = 0;
+
+  nsresult rv = mWeakSeekableInputStream->Tell(&tell);
+  if (NS_WARN_IF(NS_FAILED(rv))) {
+    return rv;
+  }
+
+  if (tell < (int64_t)mStart) {
+    *aResult = 0;
+    return NS_OK;
+  }
+
+  *aResult = tell - mStart;
+  if (*aResult > (int64_t)mLength) {
+    *aResult = mLength;
+  }
+
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+SlicedInputStream::SetEOF()
+{
+  NS_ENSURE_STATE(mInputStream);
+  NS_ENSURE_STATE(mWeakSeekableInputStream);
+
+  mClosed = true;
+  return mWeakSeekableInputStream->SetEOF();
+}
diff --git a/xpcom/io/SlicedInputStream.h b/xpcom/io/SlicedInputStream.h
index d9df2b1020e9..7864d4477e5e 100644
--- a/xpcom/io/SlicedInputStream.h
+++ b/xpcom/io/SlicedInputStream.h
@@ -11,12 +11,14 @@
 #include "nsIAsyncInputStream.h"
 #include "nsICloneableInputStream.h"
 #include "nsIIPCSerializableInputStream.h"
+#include "nsISeekableStream.h"
 
 // A wrapper for a slice of an underlying input stream.
 
 class SlicedInputStream final : public nsIAsyncInputStream
                               , public nsICloneableInputStream
                               , public nsIIPCSerializableInputStream
+                              , public nsISeekableStream
 {
 public:
   NS_DECL_THREADSAFE_ISUPPORTS
@@ -24,6 +26,7 @@ public:
   NS_DECL_NSIASYNCINPUTSTREAM
   NS_DECL_NSICLONEABLEINPUTSTREAM
   NS_DECL_NSIIPCSERIALIZABLEINPUTSTREAM
+  NS_DECL_NSISEEKABLESTREAM
 
   // Create an input stream whose data comes from a slice of aInputStream.  The
   // slice begins at aStart bytes beyond aInputStream's current position, and
@@ -53,6 +56,7 @@ private:
   // Raw pointers because these are just QI of mInputStream.
   nsICloneableInputStream* mWeakCloneableInputStream;
   nsIIPCSerializableInputStream* mWeakIPCSerializableInputStream;
+  nsISeekableStream* mWeakSeekableInputStream;
 
   uint64_t mStart;
   uint64_t mLength;