Error: Class 'Civi\\Firewall\\Firewall' not found in

My build manifest calls for the installation of: firewall@https://lab.civicrm.org/extensions/firewall/-/archive/1.1.2/firewall-1.1.2.zip

I just ran a build which resulted in success on my C-I server, but when browsing to the site I see:

The website encountered an unexpected error. Please try again later.

and in my apache2 error log I see a single entry with the following message (which I have reformatted here for ease of reading, and scrubbed of path name elements which identify my client).

[Fri Jun 11 19:57:18.360112 2021] [proxy_fcgi:error] [pid 1511:tid 140023760406272] [client 172.17.0.8:48342] 

AH01071: Got error 'PHP message: PHP Deprecated:  Array and string offset access syntax with curly braces is deprecated in /opt/local/${project_code}/files/civicrm/ext/sweetalert/sweetalert.civix.php on line 246

PHP message: Error: Class 'Civi\\Firewall\\Firewall' not found in 
/opt/local/${project_code}/files/civicrm/ext/firewall/firewall.php on line 12 

#0 /opt/local/${project_code}/drupal/vendor/civicrm/civicrm-core/CRM/Utils/Hook.php(271): firewall_civicrm_config()\n

#1 /opt/local/${project_code}/drupal/vendor/civicrm/civicrm-core/CRM/Utils/Hook/DrupalBase.php(73): CRM_Utils_Hook->runHooks()\n

#2 /opt/local/${project_code}/drupal/vendor/civicrm/civicrm-core/Civi/Core/CiviEventDispatcher.php(168): CRM_Utils_Hook_DrupalBase->invokeViaUF()\n

#3 /opt/local/${project_code}/drupal/vendor/symfony/event-dispatcher/EventDispatcher.php(214): Civi\\Core\\CiviEventDispatcher::delegateToUF()\n

#4 /opt/local/${project_code}/drupal/vendor/symfony/event-dispatcher/EventDispatcher.php(44): Symfony\\Component\\EventDispatcher\\EventDispatcher->doDispatch()\n

#5 /opt/local/${project_code}/drupal/vendor/civicrm/civicrm-core/Civi/Core/CiviEventDispatcher.php(129): Symfony\\Component\\EventDispatcher\\EventDispatcher->dispatch()\n

#6 /opt/local/${project_code}/drupal/vendor/civicrm/civicrm-core/CRM/Utils/Hook...'

A lint check at the command line reports:

root@efc9c26-00938:~# php -l /opt/local/${project_code}/files/civicrm/ext/firewall/firewall.php
No syntax errors detected in /opt/local/${project_code}/files/civicrm/ext/firewall/firewall.php

root@efc9c26-00938:~# php --version
PHP 7.4.20 (cli) (built: Jun  4 2021 21:24:37) ( NTS )
Copyright (c) The PHP Group
Zend Engine v3.4.0, Copyright (c) Zend Technologies
    with Zend OPcache v7.4.20, Copyright (c), by Zend Technologies