Use Composer-Installed twig with FuelPHP

parser.php

<?php

// Assuming composer's configured 'vendor' directory is APPPATH/vendor/, this configuration
//  file will tell fuel's parser library where to find the twig resource files.

// Place this file inside your APPPATH/config directory.

return array(
    'View_Twig' => array(
        'include' => APPPATH.'vendor'.DS.'twig'.DS.'twig'.DS.'lib'.DS.'Twig'.DS.'Autoloader.php',
    ) ,
);

Other point is to just edit the fuel/packages/parser/config/parser.php file.

The problem with that is the fuel/packages/* items are under source control from the central repo and (unless you maintain your own fork of the parser) editing these files will make your local installation diverge from fuel's. This will in turn make it more difficult to keep your fuel installation up to date.

The generally-accepted practice with fuel is to never edit any files outside your APPPATH/ directory, for this reason.