The goal of this procedure is to connect your Starlink router in bridge mode to your OPNSense firewall.
You will need the Starlink RJ45 adapter if you don't already have it to put your router in bridge mode: https://shop.starlink.com/products/fr-consumer-ethernet-adapter-gen2
Connect to the default Wi-Fi network and open the starlink application on your phone.
Now, go to Settings and enable Bypass Starlink Wi-Fi router:
Wait 5 minutes for the starlink router to restart, then connect the ethernet cable to the WAN interface of your OPNSense firewall.
Go to Interfaces -> WAN then configure the interface like this with DHCPv4.
You must untick Block private networks or you will not get any IP address since starlink user carrier grade NAT network (100.64/10).
Save interface settings.
Now, go to System -> Gateways -> Single and edit the default gateway of WAN interface:
Put the IP address of a public DNS for example in monitoring in place of the default gateway provided by Starlink to have consistent monitoring (the default gateway will be always up because she is in local):
You will finally be able to use your Starlink access: