Redundant Firewalls in an inter vlan routing configuration

[B2BMan]

I have a stacked pair of MS225 Meraki switches configured with inter-vlan routing and I need to add a redundant pair of Firewalls. I only have a single default gateway of course. Can this be solved via some sort of Meraki connectivity configuration?

[alemabrahao]

You can use a private IP on each WAN, if you can do NAT.

[GreenMan]

Your redundant firewall setup would need to involve provision of a virtual IP & MAC address operation, which either unit can adopt when it's the 'master' firewall.    You configure the default route on your routing MS225 to use that virtual IP as the next hop.    Meraki MX appliances can do this, for example, working in warm spare mode:
https://documentation.meraki.com/MX/Deployment_Guides/MX_Warm_Spare_-_High_Availability_Pair

The MX doesn't support Link Aggregation, nor actively participate in Spanning Tree, but does forward BPDUs, so you can create loops for resilience, involving your switching connecting to the MXs, which your STP setup should resolve at the switch end.  You'll see sample topologies in that document

[Boomerang94]

A single default gateway should work fine. Since you are doing routing on the MS225 stack, you will have to configure a static default route pointing towards a single IP address residing on the firewalls. Now if you are adding a redundant pair of firewalls, only one of them will be active/master at any given time which will be the owner of the IP that the switch stack is using as a gateway. There are some firewall vendors that use the (virtual IP/ virtual MAC) in their HA firewall setup (as @GreenMan  mentioned), in that case the gateway of the switches will point to the virtual IP that the HA firewall share.

As long as you have physical connectivity between the stack switches and both HA pair firewalls, STP will do it its magic and redundancy will take care of any failover scenarios.