--- a/toolkit/components/extensions/ExtensionParent.jsm
+++ b/toolkit/components/extensions/ExtensionParent.jsm
@@ -19,22 +19,22 @@ Cu.import("resource://gre/modules/Servic
 Cu.import("resource://gre/modules/XPCOMUtils.jsm");
 
 XPCOMUtils.defineLazyModuleGetter(this, "AppConstants",
                                   "resource://gre/modules/AppConstants.jsm");
 XPCOMUtils.defineLazyModuleGetter(this, "DeferredSave",
                                   "resource://gre/modules/DeferredSave.jsm");
 XPCOMUtils.defineLazyModuleGetter(this, "E10SUtils",
                                   "resource:///modules/E10SUtils.jsm");
-XPCOMUtils.defineLazyModuleGetter(this, "FileUtils",
-                                  "resource://gre/modules/FileUtils.jsm");
 XPCOMUtils.defineLazyModuleGetter(this, "MessageChannel",
                                   "resource://gre/modules/MessageChannel.jsm");
 XPCOMUtils.defineLazyModuleGetter(this, "NativeApp",
                                   "resource://gre/modules/NativeMessaging.jsm");
+XPCOMUtils.defineLazyModuleGetter(this, "OS",
+                                  "resource://gre/modules/osfile.jsm");
 XPCOMUtils.defineLazyModuleGetter(this, "PrivateBrowsingUtils",
                                   "resource://gre/modules/PrivateBrowsingUtils.jsm");
 XPCOMUtils.defineLazyModuleGetter(this, "Schemas",
                                   "resource://gre/modules/Schemas.jsm");
 
 XPCOMUtils.defineLazyServiceGetter(this, "gAddonPolicyService",
                                    "@mozilla.org/addons/policy-service;1",
                                    "nsIAddonPolicyService");
@@ -1376,23 +1376,26 @@ let IconDetails = {
   },
 };
 
 StartupCache = {
   DB_NAME: "ExtensionStartupCache",
 
   STORE_NAMES: Object.freeze(["general", "locales", "manifests", "other", "permissions", "schemas"]),
 
-  get file() {
-    return FileUtils.getFile("ProfLD", ["startupCache", "webext.sc.lz4"]);
-  },
+  file: OS.Path.join(OS.Constants.Path.localProfileDir, "startupCache", "webext.sc.lz4"),
 
   get saver() {
     if (!this._saver) {
-      this._saver = new DeferredSave(this.file.path,
+      OS.File.makeDir(OS.Path.dirname(this.file), {
+        ignoreExisting: true,
+        from: OS.Constants.Path.localProfileDir,
+      });
+
+      this._saver = new DeferredSave(this.file,
                                      () => this.getBlob(),
                                      {delay: 5000});
     }
     return this._saver;
   },
 
   async save() {
     return this.saver.saveChanges();

--- a/toolkit/components/extensions/ExtensionPermissions.jsm
+++ b/toolkit/components/extensions/ExtensionPermissions.jsm
@@ -3,38 +3,38 @@
 const {classes: Cc, interfaces: Ci, utils: Cu, results: Cr} = Components;
 
 Cu.import("resource://gre/modules/XPCOMUtils.jsm");
 
 XPCOMUtils.defineLazyModuleGetter(this, "ExtensionParent",
                                   "resource://gre/modules/ExtensionParent.jsm");
 XPCOMUtils.defineLazyModuleGetter(this, "ExtensionUtils",
                                   "resource://gre/modules/ExtensionUtils.jsm");
-XPCOMUtils.defineLazyModuleGetter(this, "FileUtils",
-                                  "resource://gre/modules/FileUtils.jsm");
 XPCOMUtils.defineLazyModuleGetter(this, "JSONFile",
                                   "resource://gre/modules/JSONFile.jsm");
+XPCOMUtils.defineLazyModuleGetter(this, "OS",
+                                  "resource://gre/modules/osfile.jsm");
 
 XPCOMUtils.defineLazyGetter(this, "StartupCache", () => ExtensionParent.StartupCache);
 
 this.EXPORTED_SYMBOLS = ["ExtensionPermissions"];
 
 const FILE_NAME = "extension-preferences.json";
 
 let prefs;
 let _initPromise;
 
 async function _lazyInit() {
-  let file = FileUtils.getFile("ProfD", [FILE_NAME]);
+  let path = OS.Path.join(OS.Constants.Path.profileDir, FILE_NAME);
 
-  prefs = new JSONFile({path: file.path});
+  prefs = new JSONFile({path});
   prefs.data = {};
 
   try {
-    let blob = await ExtensionUtils.promiseFileContents(file);
+    let blob = await ExtensionUtils.promiseFileContents(path);
     prefs.data = JSON.parse(new TextDecoder().decode(blob));
   } catch (e) {
     if (!e.becauseNoSuchFile) {
       Cu.reportError(e);
     }
   }
 }
 

--- a/toolkit/components/extensions/ExtensionUtils.jsm
+++ b/toolkit/components/extensions/ExtensionUtils.jsm
@@ -32,18 +32,18 @@ const appinfo = Cc["@mozilla.org/xre/app
 
 let nextId = 0;
 XPCOMUtils.defineLazyGetter(this, "uniqueProcessID", () => appinfo.uniqueProcessID);
 
 function getUniqueId() {
   return `${nextId++}-${uniqueProcessID}`;
 }
 
-async function promiseFileContents(file) {
-  let res = await OS.File.read(file.path);
+async function promiseFileContents(path) {
+  let res = await OS.File.read(path);
   return res.buffer;
 }
 
 
 /**
  * An Error subclass for which complete error messages are always passed
  * to extensions, rather than being interpreted as an unknown error.
  */