I am using Postman and l3FirewallRules work fine.
I removed my key and shorten the network ID
GET /api/v0/networks/N_6558366957/l7FirewallRules HTTP/1.1
Host: api.meraki.com
X-Cisco-Meraki-API-Key: XXXXX
Content-Type: application/json
cache-control: no-cache
Postman-Token: f217d697-1c46-4dee-bec2-1a97383ccff4