Since version 4.7.2, you can customize the subject and body of all email notifications sent by NinjaFirewall WP/WP+ Edition.

NinjaFirewall WP Edition (free version)

You will find the two templates inside the plugin’s /lib/ folder:

• wp-content/plugins/ninjafirewall/lib/mail_template_firewall.php: it includes notifications sent by the firewall before WordPress loads. As such, internationalization is not available for those strings.
• wp-content/plugins/ninjafirewall/lib/mail_template_plugin.php: it includes notifications sent by the plugin part of firewall and internationalization is available for those strings.

NinjaFirewall WP+ Edition (premium version)

You will find the two templates inside the plugin’s /lib/ folder, with slightly different names:

• wp-content/plugins/nfwplus/lib/mail_template_firewall_pro.php: it includes notifications sent by the firewall before WordPress loads. As such, internationalization is not available for those strings.
• wp-content/plugins/nfwplus/lib/mail_template_plugin_pro.php: it includes notifications sent by the plugin part of firewall and internationalization is available for those strings.

To customize them, simply copy the two files into the wp-content/nfwlog/ folder (that’s the firewall log and cache folder) and edit them. NinjaFirewall will detect and load them.

Do not remove the original files: NinjaFirewall will load them first and then will load your customized templates.