Hello,
You are experiencing asymmetric routing in your network. Firewall blocks TCP
traffic when it does not see bidirectional traffic through itself. If you
are running 8.2 code version, you can configure TCP State bypassing on the
inside interface. If you running code prior to 8.2, then I think the best
solution would be to swap the IP's of the ASA and the MPLS router i.e. let
the MPLS router have the IP of .254 and ASA .253. On the MPLS router, have
the default gateway as .253.
Here is the link that explains TCP State Bypass configuration.
http://www.cisco.com/en/US/docs/security/asa/asa82/configuration/guide/conns
_tcpstatebypass.pdf
Hope this helps.
Regards,
NT