you could do one of the following ways to implement Internet access for L3 MPLS VPN
1. using a separate PE interface in global routing table: in this case the FWSM and an interface in the PE/PEs will require to be in the the global routing table to have the Internet access and then you can inject that route to the VRF/VRFs
2. Internet access using route leaking between VRFs and the global route table: by using this method you will need to configure a static default route with a next hop as an Internet gateway in your case the FWSM, reachable through the global routing table, this VRF default route need to be injected/redistributed in the PE-CE routing (MP-BGP) to provide the outbound Internet connectivity to your VRFs.
inbound traffic from Internet will require either NATed VRF or a static routes from the global routing table points to the VRF interface
3. the other method is the used of shared service: with this method you need to put the Internet service FWSM in its own VRF then you can control the import and export between the Internet VRF and other VRFs through import/export of the VRFs route-target values
good luck
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