I also had a look at how you would implement multiple ISP load-balancing on an edgerouter. The following config instrcutions are what I came up with:
1: Firewall rule to use load-balance group (we can restrict to specific traffic)
set firewall modify rule 1 action modify
set firewall modify rule 1 modify lb-group
2: Create a load-balance group
set load-balance group interface eth0 failover-only
set load-balance group interface eth0 route-test initial-delay 60
set load-balance group interface eth0 route-test interval 10
set load-balance group interface eth0 route-test type ping target 8.8.8.8
set load-balance group interface eth1 route-test initial-delay 60
set load-balance group interface eth1 route-test interval 10
set load-balance group interface eth1 route-test type ping target 8.8.8.8
set load-balance group lb-local enable
set load-balance group lb-local-metric-change disable
3: Apply the firewall rule to all LAN interfaces that need it
set interfaces switch switch0 firewall in modify