diff --git a/toolkit/components/extensions/ExtensionStorageSync.jsm b/toolkit/components/extensions/ExtensionStorageSync.jsm
index d82d4edb9ee9..ba64ea462ca3 100644
--- a/toolkit/components/extensions/ExtensionStorageSync.jsm
+++ b/toolkit/components/extensions/ExtensionStorageSync.jsm
@@ -304,9 +304,6 @@ function cleanUpForContext(extension, context) {
  * @returns {Promise<Collection>}
  */
 const openCollection = Task.async(function* (extension, context) {
-  // FIXME: This leaks metadata about what extensions a user has
-  // installed.  We should calculate collection ID using a hash of
-  // user ID, extension ID, and some secret.
   let collectionId = extension.id;
   const {kinto} = yield storageSyncInit;
   const coll = kinto.collection(collectionId, {
@@ -366,8 +363,7 @@ this.ExtensionStorageSync = {
       log.info("User was not signed into FxA; cannot sync");
       throw new Error("Not signed in to FxA");
     }
-    // FIXME: this leaks metadata about what extensions are being used
-    const collectionId = extension.id;
+    const collectionId = extensionIdToCollectionId(signedInUser, extension.id);
     let syncResults;
     try {
       syncResults = yield this._syncCollection(collection, {
diff --git a/toolkit/components/extensions/test/xpcshell/test_ext_storage_sync.js b/toolkit/components/extensions/test/xpcshell/test_ext_storage_sync.js
index b0df55f88499..b449848360ad 100644
--- a/toolkit/components/extensions/test/xpcshell/test_ext_storage_sync.js
+++ b/toolkit/components/extensions/test/xpcshell/test_ext_storage_sync.js
@@ -328,8 +328,6 @@ function assertKeyRingKey(keyRing, extensionId, expectedKey, message) {
 
 // Tests using this ID will share keys in local storage, so be careful.
 const defaultExtensionId = "{13bdde76-4dc7-11e6-9bdc-54ee758d6342}";
-// FIXME: need to access whatever mechanism we use in the syncing code
-const defaultCollectionId = defaultExtensionId;
 const defaultExtension = {id: defaultExtensionId};
 
 const BORING_KB = "0123456789abcdef0123456789abcdef0123456789abcdef0123456789abcdef";
@@ -343,6 +341,7 @@ const loggedInUser = {
     },
   },
 };
+const defaultCollectionId = extensionIdToCollectionId(loggedInUser, defaultExtensionId);
 
 function uuid() {
   const uuidgen = Cc["@mozilla.org/uuid-generator;1"].getService(Ci.nsIUUIDGenerator);