try concurrency 1

Author: hsubox76

packages/firestore/karma.conf.js

@@ -24,7 +24,9 @@ module.exports = function (config) {
     browsers: getTestBrowsers(argv),
     // files to load into karma
     files: getTestFiles(argv),
-
+    
+    concurrency: 1,
+    
     preprocessors: {
       'test/**/*.ts': ['webpack', 'sourcemap']
     },