Hi,
You can probably configure VPLS between 2 MX240s and call irb as a routing-interface inside that VPLS instance. Also you can then configure 2 irbs on MX240 and reth(irb) interface on firewall with the same LAN segment and run any IGP for route exchange. The interface going towards firewall can be L2 interface which will be part of VPLS instance.
This way, even if one MX goes down, firewall will have IGP reachability over irb to the other MX and continue to forward traffic.
Thanks