--- a/toolkit/modules/subprocess/subprocess_worker_win.js
+++ b/toolkit/modules/subprocess/subprocess_worker_win.js
@@ -516,17 +516,20 @@ class Process extends BaseProcess {
       ok = libc.SetInformationJobObject(this.jobHandle, win32.JobObjectExtendedLimitInformation,
                                         ctypes.cast(info.address(), ctypes.voidptr_t),
                                         info.constructor.size);
       errorMessage = `Failed to set job limits: 0x${(ctypes.winLastError || 0).toString(16)}`;
     }
 
     if (ok) {
       ok = libc.AssignProcessToJobObject(this.jobHandle, procInfo.hProcess);
-      errorMessage = `Failed to attach process to job object: 0x${(ctypes.winLastError || 0).toString(16)}`;
+      if (!ok) {
+        libc.TerminateProcess(procInfo.hProcess, TERMINATE_EXIT_CODE);
+        errorMessage = `Failed to attach process to job object: 0x${(ctypes.winLastError || 0).toString(16)}`;
+      }
     }
 
     if (!ok) {
       for (let pipe of this.pipes) {
         pipe.close();
       }
       throw new Error(errorMessage);
     }