diff --git a/xpcom/io/nsStringStream.cpp b/xpcom/io/nsStringStream.cpp
index 511f2b9034b1..4f687e257bbc 100644
--- a/xpcom/io/nsStringStream.cpp
+++ b/xpcom/io/nsStringStream.cpp
@@ -22,6 +22,7 @@
 #include "nsIClassInfoImpl.h"
 #include "mozilla/Attributes.h"
 #include "mozilla/ipc/InputStreamUtils.h"
+#include "mozilla/ReentrantMonitor.h"
 #include "nsIIPCSerializableInputStream.h"
 
 using namespace mozilla::ipc;
@@ -48,7 +49,7 @@ class nsStringInputStream final : public nsIStringInputStream,
   NS_DECL_NSIIPCSERIALIZABLEINPUTSTREAM
   NS_DECL_NSICLONEABLEINPUTSTREAM
 
-  nsStringInputStream() : mOffset(0) { Clear(); }
+  nsStringInputStream() : mOffset(0), mMon("nsStringInputStream") { Clear(); }
 
   nsresult Init(nsCString&& aString);
 
@@ -69,9 +70,13 @@ class nsStringInputStream final : public nsIStringInputStream,
 
   nsDependentCSubstring mData;
   uint32_t mOffset;
+
+  mozilla::ReentrantMonitor mMon;
 };
 
 nsresult nsStringInputStream::Init(nsCString&& aString) {
+  ReentrantMonitorAutoEnter lock(mMon);
+
   if (!mData.Assign(std::move(aString), fallible)) {
     return NS_ERROR_OUT_OF_MEMORY;
   }
@@ -109,6 +114,8 @@ nsStringInputStream::GetType(uint16_t* aType) {
 
 NS_IMETHODIMP
 nsStringInputStream::GetData(nsACString& data) {
+  ReentrantMonitorAutoEnter lock(mMon);
+
   // The stream doesn't have any data when it is closed.  We could fake it
   // and return an empty string here, but it seems better to keep this return
   // value consistent with the behavior of the other 'getter' methods.
@@ -122,6 +129,8 @@ nsStringInputStream::GetData(nsACString& data) {
 
 NS_IMETHODIMP
 nsStringInputStream::SetData(const nsACString& aData) {
+  ReentrantMonitorAutoEnter lock(mMon);
+
   if (NS_WARN_IF(!mData.Assign(aData, fallible))) {
     return NS_ERROR_OUT_OF_MEMORY;
   }
@@ -142,6 +151,8 @@ nsStringInputStream::ToString(char** aResult) {
 
 NS_IMETHODIMP
 nsStringInputStream::SetData(const char* aData, int32_t aDataLen) {
+  ReentrantMonitorAutoEnter lock(mMon);
+
   if (NS_WARN_IF(!aData)) {
     return NS_ERROR_INVALID_ARG;
   }
@@ -156,6 +167,8 @@ nsStringInputStream::SetData(const char* aData, int32_t aDataLen) {
 
 NS_IMETHODIMP
 nsStringInputStream::AdoptData(char* aData, int32_t aDataLen) {
+  ReentrantMonitorAutoEnter lock(mMon);
+
   if (NS_WARN_IF(!aData)) {
     return NS_ERROR_INVALID_ARG;
   }
@@ -166,6 +179,8 @@ nsStringInputStream::AdoptData(char* aData, int32_t aDataLen) {
 
 NS_IMETHODIMP
 nsStringInputStream::ShareData(const char* aData, int32_t aDataLen) {
+  ReentrantMonitorAutoEnter lock(mMon);
+
   if (NS_WARN_IF(!aData)) {
     return NS_ERROR_INVALID_ARG;
   }
@@ -181,6 +196,8 @@ nsStringInputStream::ShareData(const char* aData, int32_t aDataLen) {
 
 NS_IMETHODIMP_(size_t)
 nsStringInputStream::SizeOfIncludingThis(MallocSizeOf aMallocSizeOf) {
+  ReentrantMonitorAutoEnter lock(mMon);
+
   size_t n = aMallocSizeOf(this);
   n += mData.SizeOfExcludingThisIfUnshared(aMallocSizeOf);
   return n;
@@ -192,12 +209,16 @@ nsStringInputStream::SizeOfIncludingThis(MallocSizeOf aMallocSizeOf) {
 
 NS_IMETHODIMP
 nsStringInputStream::Close() {
+  ReentrantMonitorAutoEnter lock(mMon);
+
   Clear();
   return NS_OK;
 }
 
 NS_IMETHODIMP
 nsStringInputStream::Available(uint64_t* aLength) {
+  ReentrantMonitorAutoEnter lock(mMon);
+
   NS_ASSERTION(aLength, "null ptr");
 
   if (Closed()) {
@@ -217,6 +238,8 @@ nsStringInputStream::Read(char* aBuf, uint32_t aCount, uint32_t* aReadCount) {
 NS_IMETHODIMP
 nsStringInputStream::ReadSegments(nsWriteSegmentFun aWriter, void* aClosure,
                                   uint32_t aCount, uint32_t* aResult) {
+  ReentrantMonitorAutoEnter lock(mMon);
+
   NS_ASSERTION(aResult, "null ptr");
   NS_ASSERTION(Length() >= mOffset, "bad stream state");
 
@@ -258,6 +281,8 @@ nsStringInputStream::IsNonBlocking(bool* aNonBlocking) {
 
 NS_IMETHODIMP
 nsStringInputStream::Seek(int32_t aWhence, int64_t aOffset) {
+  ReentrantMonitorAutoEnter lock(mMon);
+
   if (Closed()) {
     return NS_BASE_STREAM_CLOSED;
   }
@@ -289,6 +314,8 @@ nsStringInputStream::Seek(int32_t aWhence, int64_t aOffset) {
 
 NS_IMETHODIMP
 nsStringInputStream::SetEOF() {
+  ReentrantMonitorAutoEnter lock(mMon);
+
   if (Closed()) {
     return NS_BASE_STREAM_CLOSED;
   }
@@ -303,6 +330,8 @@ nsStringInputStream::SetEOF() {
 
 NS_IMETHODIMP
 nsStringInputStream::Tell(int64_t* aOutWhere) {
+  ReentrantMonitorAutoEnter lock(mMon);
+
   if (Closed()) {
     return NS_BASE_STREAM_CLOSED;
   }
@@ -352,6 +381,8 @@ void nsStringInputStream::SerializeInternal(InputStreamParams& aParams,
                                             bool aDelayedStart,
                                             uint32_t aMaxSize,
                                             uint32_t* aSizeUsed, M* aManager) {
+  ReentrantMonitorAutoEnter lock(mMon);
+
   MOZ_ASSERT(aSizeUsed);
   *aSizeUsed = 0;
 
@@ -397,6 +428,8 @@ nsStringInputStream::GetCloneable(bool* aCloneableOut) {
 
 NS_IMETHODIMP
 nsStringInputStream::Clone(nsIInputStream** aCloneOut) {
+  ReentrantMonitorAutoEnter lock(mMon);
+
   RefPtr<nsStringInputStream> ref = new nsStringInputStream();
   nsresult rv = ref->SetData(mData);
   if (NS_WARN_IF(NS_FAILED(rv))) {