firewall interferes with addon domains running php7

Depending on your server configuration, this problem can indeed happen. There are a few options though. The first one I recommend is to used the .htninja user script to tell the firewall to filter only the main domain.
Assuming that your main site is “domain.tld” or “www.domain.tld”, you could use this code:

<?php
/*
 +===================================================================+
 | NinjaFirewall optional configuration file                         |
 |                                                                   |
 | See: https://nintechnet.com/ninjafirewall/wp-edition/help/?htninja|
 +===================================================================+
*/

// If the requested host is not "domain.tld", ignore it:
if ( stripos( $_SERVER['HTTP_HOST'], 'domain.tld') === FALSE ) {
   return 'ALLOW';
}

Note that it will match “domain.tld”, “www.domain.tld”, “WWW.DOMAIN.TLD”, but also “sub.domain.tld”.