Switch Loop Prevention

[OneOfUs]

I am trying to design a new FGT-1500D deployment to replace a Juniper firewall.  The firewall connects to a pair of switches in one computer room and another pair of switches in another computer room.  The Juniper is using redundant links to ensure only one up-link is active to each switch pair.

             ____ [Switch A]             /                >--- {Server 1}            /  ___ [Switch B]           /__/ [Firewall] __           \  \___ [Switch C]            \                 >--- {Server 2}             \____ [Switch D]

FIGURE1: ASCII Topology

Going forward I would like to span a VLAN between both computer rooms.  other than using a software switch and using spanning-tree are there other options I should consider (EAPS, MLAG, LACP, etc...)?

There is no connectivity between the switch pairs (though it could be added).  I would like to do this with the minimum cabling required, the firewalls will be HA(a-p) and the distance between computer rooms requires fiber runs. 

Worth noting, the servers attached to the switch is what effectively would create a loop.  Any thoughts would be appreciated.