When looking at the image, the Switch is routing capable (L3) and is the default gateway for clients in the LAN. We plan to introduce a new Firewall and stand before the question if we should give its LAN interface an IP address of the LAN subnet or if we should create a separate transfer subnet between Routing Switch and Firewall? Are there any pros/cons or does it not matter?
An L3 switch usually routes at wire speed, so pointing the clients' default gateway to the core switch doesn't cost you anything (ie. no extra L2 hop). That default gateway is also advantageous if there are other gateways leading elsewhere (security zones, other buildings, ...) that you don't want to bother the firewall with.
Connecting the firewall doesn't necessarily require a transfer subnet. You can just as well put in on the same subnet as the core switch - unless you don't want the clients to connect directly, of course.
For maximum flexibility, I'd trunk all VLANs to the firewall. Then you can set (and allow) the default gateway that works best for your performance and security requirements (and possibly deny/filter the other gateway/router).
If default gateway of clients is configured on switch . Then you can use another ip address to configure on interfaces connecting firewall and switch and ensure
specific route Configured in firewall pointing towards. Switch ingress interface.
default route need to configure on switch pointing towards ingress interfàce of firewall .
But make sure your switch is layer3 switch . So that ip address can assigned to interfaces connecting switch and firewall and you can even create Vlans and isolate traffic among VLANs in future .