--- a/browser/components/migration/FirefoxProfileMigrator.js
+++ b/browser/components/migration/FirefoxProfileMigrator.js
@@ -56,17 +56,17 @@ FirefoxProfileMigrator.prototype._getAll
 };
 
 function sorter(a, b) {
   return a.id.toLocaleLowerCase().localeCompare(b.id.toLocaleLowerCase());
 }
 
 Object.defineProperty(FirefoxProfileMigrator.prototype, "sourceProfiles", {
   get: function() {
-    return this._getAllProfiles().keys().map(x => ({id: x, name: x})).sort(sorter);
+    return [...this._getAllProfiles().keys()].map(x => ({id: x, name: x})).sort(sorter);
   }
 });
 
 FirefoxProfileMigrator.prototype._getFileObject = function(dir, fileName) {
   let file = dir.clone();
   file.append(fileName);
 
   // File resources are monolithic.  We don't make partial copies since