--- a/browser/components/sessionstore/SessionFile.jsm
+++ b/browser/components/sessionstore/SessionFile.jsm
@@ -34,16 +34,18 @@ Cu.import("resource://gre/modules/Servic
 Cu.import("resource://gre/modules/XPCOMUtils.jsm");
 Cu.import("resource://gre/modules/osfile.jsm");
 Cu.import("resource://gre/modules/Promise.jsm");
 Cu.import("resource://gre/modules/AsyncShutdown.jsm");
 Cu.import("resource://gre/modules/Preferences.jsm");
 
 XPCOMUtils.defineLazyModuleGetter(this, "console",
   "resource://gre/modules/Console.jsm");
+XPCOMUtils.defineLazyModuleGetter(this, "PromiseUtils",
+  "resource://gre/modules/PromiseUtils.jsm");
 XPCOMUtils.defineLazyModuleGetter(this, "RunState",
   "resource:///modules/sessionstore/RunState.jsm");
 XPCOMUtils.defineLazyModuleGetter(this, "TelemetryStopwatch",
   "resource://gre/modules/TelemetryStopwatch.jsm");
 XPCOMUtils.defineLazyModuleGetter(this, "Task",
   "resource://gre/modules/Task.jsm");
 XPCOMUtils.defineLazyServiceGetter(this, "Telemetry",
   "@mozilla.org/base/telemetry;1", "nsITelemetry");
@@ -174,16 +176,20 @@ var SessionFileInternal = {
       return order;
     },
   }),
 
   // `true` once `write` has succeeded at last once.
   // Used for error-reporting.
   _hasWriteEverSucceeded: false,
 
+  // Resolved once initialization is complete.
+  // The promise never rejects.
+  _deferredInitialized: PromiseUtils.defer(),
+
   // The ID of the latest version of Gecko for which we have an upgrade backup
   // or |undefined| if no upgrade backup was ever written.
   get latestUpgradeBackupID() {
     try {
       return Services.prefs.getCharPref(PREF_UPGRADE_BACKUP);
     } catch (ex) {
       return undefined;
     }
@@ -250,22 +256,24 @@ var SessionFileInternal = {
         parsed: null
       };
     }
 
     result.noFilesFound = noFilesFound;
 
     // Initialize the worker to let it handle backups and also
     // as a workaround for bug 964531.
-    SessionWorker.post("init", [result.origin, this.Paths, {
+    let initialized = SessionWorker.post("init", [result.origin, this.Paths, {
       maxUpgradeBackups: Preferences.get(PREF_MAX_UPGRADE_BACKUPS, 3),
       maxSerializeBack: Preferences.get(PREF_MAX_SERIALIZE_BACK, 10),
       maxSerializeForward: Preferences.get(PREF_MAX_SERIALIZE_FWD, -1)
     }]);
 
+    initialized.catch(Promise.reject).then(() => this._deferredInitialized.resolve());
+
     return result;
   }),
 
   write: function (aData) {
     if (RunState.isClosed) {
       return Promise.reject(new Error("SessionFile is closed"));
     }
 
@@ -276,17 +284,17 @@ var SessionFileInternal = {
       isFinalWrite = true;
       RunState.setClosed();
     }
 
     let performShutdownCleanup = isFinalWrite &&
       !sessionStartup.isAutomaticRestoreEnabled();
 
     let options = {isFinalWrite, performShutdownCleanup};
-    let promise = SessionWorker.post("write", [aData, options]);
+    let promise = this._deferredInitialized.promise.then(() => SessionWorker.post("write", [aData, options]));
 
     // Wait until the write is done.
     promise = promise.then(msg => {
       // Record how long the write took.
       this._recordTelemetry(msg.telemetry);
       this._hasWriteEverSucceeded = true;
       if (msg.result.upgradeBackup) {
         // We have just completed a backup-on-upgrade, store the information
@@ -323,17 +331,17 @@ var SessionFileInternal = {
 
       if (isFinalWrite) {
         Services.obs.notifyObservers(null, "sessionstore-final-state-write-complete", "");
       }
     });
   },
 
   wipe: function () {
-    return SessionWorker.post("wipe");
+    return this._deferredInitialized.promise.then(() => SessionWorker.post("wipe"));
   },
 
   _recordTelemetry: function(telemetry) {
     for (let id of Object.keys(telemetry)){
       let value = telemetry[id];
       let samples = [];
       if (Array.isArray(value)) {
         samples.push(...value);