pin firefox to to v 64

scripts/karma.conf.js

@@ -37,5 +37,7 @@ module.exports = function(config) {
 
   config = generate(config, options);
 
-  // any other custom stuff not supported by options here!
+  // pin Browserstack Firefox version to 64
+  // eslint-disable-next-line camelcase
+  config.customLaunchers.bsFirefox.browser_version = '64.0';
 };