class ProjectFactory extends \Kreait\Firebase\Symfony\Bundle\DependencyInjection\Factory\ProjectFactory
{
    public function createFactory(array $config = []): Factory
    {
        $factory = parent::createFactory($config);

        $httpClientOptions = HttpClientOptions::default();
        $httpClientOptions = $httpClientOptions->withGuzzleConfigOptions([
            'proxy' => 'http://proxy',
        ]);

        return $factory->withHttpClientOptions($httpClientOptions);
    }
}

    ####
    # Kreait Firebase
    ####
    kreait_firebase.http_config_options:
        class: Kreait\Firebase\Http\HttpClientOptions
        factory: ['Kreait\Firebase\Http\HttpClientOptions', 'default']
        calls:
            - withProxy: !returns_clone ['%env(HDS_HTTP_PROXY)%']

    Kreait\Firebase\Factory:
        calls:
            - withHttpClientOptions: !returns_clone ['@kreait_firebase.http_config_options']
            - withServiceAccount: !returns_clone ['%env(resolve:FIREBASE_CREDENTIALS_PATH)%']
            - withHttpLogger: !returns_clone ['@monolog.logger.firebase']

    Kreait\Firebase\Contract\Messaging:
        factory: ['@Kreait\Firebase\Factory', 'createMessaging']