--- a/toolkit/components/extensions/NativeMessaging.jsm
+++ b/toolkit/components/extensions/NativeMessaging.jsm
@@ -425,18 +425,18 @@ this.NativeApp = class extends EventEmit
 
     port = Cu.cloneInto(port, this.context.cloneScope, {cloneFunctions: true});
 
     return port;
   }
 
   sendMessage(msg) {
     let responsePromise = new Promise((resolve, reject) => {
-      this.on("message", (what, msg) => { resolve(msg); });
-      this.on("disconnect", (what, err) => { reject(err); });
+      this.once("message", (what, msg) => { resolve(msg); });
+      this.once("disconnect", (what, err) => { reject(err); });
     });
 
     let result = this.startupPromise.then(() => {
       this.send(msg);
       return responsePromise;
     });
 
     result.then(() => {