fromJson

Restore a `FirefoxAccount` instance from serialized state.

Given a JSON string previously obtained from `FirefoxAccount::to_json`, this method will deserialize it and return a live `FirefoxAccount` instance.

⚠️ Warning: since the serialized state contains access tokens, you should not call from_json multiple times on the same data. This would result in multiple live objects sharing the same access tokens and is likely to produce unexpected behaviour.