--- a/toolkit/modules/subprocess/subprocess_worker_common.js
+++ b/toolkit/modules/subprocess/subprocess_worker_common.js
@@ -144,16 +144,20 @@ let requests = {
     return {data: new Set(io.pipes.keys())};
   },
 
   getProcesses() {
     let data = new Map(Array.from(io.processes.values(),
                                   proc => [proc.id, proc.pid]));
     return {data};
   },
+
+  waitForNoProcesses() {
+    return Promise.all(Array.from(io.processes.values(), proc => proc.exitPromise));
+  },
 };
 
 onmessage = event => {
   let {msg, msgId, args} = event.data;
 
   new Promise(resolve => {
     resolve(requests[msg](...args));
   }).then(result => {