You will need two routes:
1) Internet router IP as default gateway (may be automatic depending on WAN ip setup).
2) MPLS router IP as next hop to Site B subnet (static route).
Everything else will be directly connected so routes will be automatically created.
Once you have the routes in place, the firewall policies are simple.
Outbound Internet traffic will require the following firewall policies:
1) Src: Site A, Dst: Internet, NAT enabled.
2) Src: Site B, Dst: Internet, NAT enabled.
Site-to-Site traffic will require the following firewall policies:
1) Src: Site A, Dst: Site B, NAT disabled.
2) Src: Site B, Dst: Site A, NAT disabled.
Limit protocols and addresses as you see fit.