--- a/toolkit/components/extensions/ExtensionStorageSync.jsm
+++ b/toolkit/components/extensions/ExtensionStorageSync.jsm
@@ -299,19 +299,16 @@ function cleanUpForContext(extension, co
  *                    be opened.
  * @param {Context} context
  *                  The context for this extension. The Collection
  *                  will shut down automatically when all contexts
  *                  close.
  * @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, {
     idSchema: storageSyncIdSchema,
     remoteTransformers: [new CollectionKeyEncryptionRemoteTransformer(extension.id)],
   });
   return coll;
 });
@@ -361,18 +358,17 @@ this.ExtensionStorageSync = {
 
   sync: Task.async(function* (extension, collection) {
     const signedInUser = yield this._fxaService.getSignedInUser();
     if (!signedInUser) {
       // FIXME: this should support syncing to self-hosted
       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, {
         strategy: "client_wins",
         collection: collectionId,
       });
     } catch (err) {
       log.warn("Syncing failed", err);

--- a/toolkit/components/extensions/test/xpcshell/test_ext_storage_sync.js
+++ b/toolkit/components/extensions/test/xpcshell/test_ext_storage_sync.js
@@ -323,31 +323,30 @@ function assertKeyRingKey(keyRing, exten
   ok(keyRing.hasKeysFor([extensionId]),
      `expected keyring to have a key for ${extensionId}\n`);
   deepEqual(keyRing.keyForCollection(extensionId).keyPairB64, expectedKey.keyPairB64,
             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";
 const ANOTHER_KB = "0123456789abcdef0123456789abcdef0123456789abcdef0123456789abcde0";
 const loggedInUser = {
   uid: "0123456789abcdef0123456789abcdef",
   kB: BORING_KB,
   oauthTokens: {
     "sync:addon-storage": {
       token: "some-access-token",
     },
   },
 };
+const defaultCollectionId = extensionIdToCollectionId(loggedInUser, defaultExtensionId);
 
 function uuid() {
   const uuidgen = Cc["@mozilla.org/uuid-generator;1"].getService(Ci.nsIUUIDGenerator);
   return uuidgen.generateUUID();
 }
 
 add_task(function* test_key_to_id() {
   equal(keyToId("foo"), "key-foo");