check bsFirefox exists before modifying

Author: Essk

scripts/karma.conf.js

@@ -28,8 +28,10 @@ module.exports = function(config) {
         defaults = {};
       }
       // pin Browserstack Firefox version to 64
-      // eslint-disable-next-line camelcase
-      defaults.bsFirefox.browser_version = '64.0';
+      if (defaults.bsFirefox) {
+        // eslint-disable-next-line camelcase
+        defaults.bsFirefox.browser_version = '64.0';
+      }
 
       return defaults;
     },