browser.php
public function useProxy($proxy, $username = false, $password = false)
    {
        $this->user_agent->useProxy($proxy, $username, $password);
    }
public function useProxy($proxy, $username = false, $password = false)
    {
        $this->user_agent->useProxy($proxy, $username, $password);
    }
Sets proxy to use on all requests for when testing from behind a firewall.
Set URL to false to disable.

@param string $proxy        Proxy URL.
@param string $username     Proxy username for authentication.
@param string $password     Proxy password for authentication.