Performing HA or failover at layer 2 creates a layer 2 loop. You have to rely on STP or similar protocols to stop the loop and then switch the ports if the preferred path fails.
It's a mess, an accident waiting to happen, and difficult to get right because a lot of it depends on your switches and not the firewall at all.
You can certainly try it if you like, but it's not something we like to encourage. :-)