I don't think what you want is possible by disabling firewall/NAT.
With a bridge you would have on your LAN side the public subnet from the WAN.
When you want failover you also need firewall rules, since the firewall rules determine to which gateway (or in your case failover-pool) the frames are sent.
–> You need to enable the firewall/NAT part again.
Follow the guides on loadbalancing/failover on the wiki.
http://doc.pfsense.com/index.php/MultiWanVersion1.2