Hi,
You can configure IRB as routing-interface on both MX routers. You can use different IPs but put all the IRBs in the same subnet.
Something like this:
MX1---irb.1(10.10.10.1/24)
| \ ____ reth(10.10.10.3/24) Firewall
| /
MX2--irb.1(10.10.10.2/24)
You can then configure IGP between the routers and firewall and play with metric to prefer any one MX.
Thanks