diff --git a/toolkit/modules/subprocess/subprocess_shared_win.js b/toolkit/modules/subprocess/subprocess_shared_win.js
index 8fc98ac97c5..fcdc7277aea 100644
--- a/toolkit/modules/subprocess/subprocess_shared_win.js
+++ b/toolkit/modules/subprocess/subprocess_shared_win.js
@@ -88,6 +88,7 @@ Object.assign(win32, {
   ERROR_BROKEN_PIPE: 109,
   ERROR_INSUFFICIENT_BUFFER: 122,
   ERROR_ABANDONED_WAIT_0: 735,
+  ERROR_IO_INCOMPLETE: 996,
   ERROR_IO_PENDING: 997,
 
   FILE_ATTRIBUTE_NORMAL: 0x00000080,
diff --git a/toolkit/modules/subprocess/subprocess_win.worker.js b/toolkit/modules/subprocess/subprocess_win.worker.js
index c7672828d1d..6186e70b588 100644
--- a/toolkit/modules/subprocess/subprocess_win.worker.js
+++ b/toolkit/modules/subprocess/subprocess_win.worker.js
@@ -105,11 +105,20 @@ class Pipe extends BasePipe {
       debug(`Failed to associate IOCP: ${ctypes.winLastError}`);
     }
 
+    // this.buffer is set to an ArrayBuffer, which should not be reused nor
+    // released until the IO methods (ReadFile or WriteFile) have acknowledged
+    // completion, or reported an error other than ERROR_IO_PENDING.
     this.buffer = null;
+    // Whether this.buffer is part of a pending IO operation.
+    this.bufferIsPendingIO = false;
+    // When close(force = true) is called while IO is pending, we notify
+    // read()/write() callers of completion but internally we await a IOCP
+    // message for this pipe before closing the pipe for real.
+    this.awaitingBufferClose = false;
   }
 
   hasPendingIO() {
-    return !!this.pending.length;
+    return !!this.pending.length || this.bufferIsPendingIO;
   }
 
   maybeClose() {}
@@ -138,6 +147,12 @@ class Pipe extends BasePipe {
     }
     this.pending.length = 0;
 
+    if ((this.bufferIsPendingIO &&= this.#checkIfBufferIsStillPendingIO())) {
+      // We cannot release the pipe (specifically this.buffer) until the
+      // pending ReadFile/WriteFile operation on the buffer completed.
+      this.awaitingBufferClose = true;
+      return this.closedPromise;
+    }
     this.buffer = null;
 
     if (!this.closed) {
@@ -161,6 +176,18 @@ class Pipe extends BasePipe {
   onError() {
     this.close(true);
   }
+
+  #checkIfBufferIsStillPendingIO() {
+    let numberOfBytesTransferred = win32.DWORD();
+    let ok = libc.GetOverlappedResult(
+      this.handle,
+      this.overlapped.address(),
+      numberOfBytesTransferred.address(),
+      false
+    );
+    // Ok or error other than ERROR_IO_INCOMPLETE means that I/O completed.
+    return !ok && ctypes.winLastError === win32.ERROR_IO_INCOMPLETE;
+  }
 }
 
 class InputPipe extends Pipe {
@@ -189,7 +216,7 @@ class InputPipe extends Pipe {
         false
       );
 
-      if (!ok) {
+      if (!ok && ctypes.winLastError !== win32.ERROR_IO_INCOMPLETE) {
         this.onError();
       }
     }
@@ -226,6 +253,7 @@ class InputPipe extends Pipe {
    */
   readBuffer(count) {
     this.buffer = new ArrayBuffer(count);
+    this.bufferIsPendingIO = true;
 
     let ok = libc.ReadFile(
       this.handle,
@@ -239,6 +267,7 @@ class InputPipe extends Pipe {
       !ok &&
       (!this.process.handle || ctypes.winLastError !== win32.ERROR_IO_PENDING)
     ) {
+      this.bufferIsPendingIO = ctypes.winLastError === win32.ERROR_IO_PENDING;
       this.onError();
     } else {
       io.updatePollEvents();
@@ -326,6 +355,7 @@ class OutputPipe extends Pipe {
    */
   writeBuffer(buffer) {
     this.buffer = buffer;
+    this.bufferIsPendingIO = true;
 
     let ok = libc.WriteFile(
       this.handle,
@@ -336,6 +366,7 @@ class OutputPipe extends Pipe {
     );
 
     if (!ok && ctypes.winLastError !== win32.ERROR_IO_PENDING) {
+      this.bufferIsPendingIO = false;
       this.onError();
     } else {
       io.updatePollEvents();
@@ -849,7 +880,8 @@ io = {
           debug(`IOCP notification for unknown pipe: ${pipeId}`);
           continue;
         }
-        if (deqWinErr === win32.ERROR_BROKEN_PIPE) {
+        pipe.bufferIsPendingIO = false;
+        if (deqWinErr === win32.ERROR_BROKEN_PIPE || pipe.awaitingBufferClose) {
           pipe.onError();
           continue;
         }