Changes

Finally, the server-provided wrap(kB) value is simply XORed with the password-derived wrapKey (both are 32-byte strings) to obtain kB. There is no MAC on wrap(kB).

"kA" and "kB" enable the browser to encrypt/decrypt synchronized data records. They will be used to derive separate encryption and HMAC keys for each data collection (bookmarks, form-fill data, saved-password, open-tabs, etc). This will allow the user to share some data, but not everything, with a third party. The client may intentionally forget kA and kB (only retaining the derived keys) to reduce the power available to someone who steals their device.