I am a software developer, so I am not a complete fool when it comes
to computers, however I was placed in charge of handling an ISP change
at our company (we are very small, can't afford a real network admin).
I've done some networking before, so I'm familiar with routers,
gateways, etc. Here is the situation (numbers are fake):

The new ISP sent me the standard information. They gave me:
Assigned netblock: 111.222.333.444/28
Customer WAN: 555.666.777.888/30
Gateway: 555.666.777.887
Customer LAN: 111.222.333.445/28
LAN Gateway: 111.222.333.445/28

My main question is what is the difference between the Customer WAN
and Customer LAN blocks? What address do I use as the external
interface for our WatchGuard firewall? I plugged a laptop directly
into the new link, and I know I must use the 555.666.777.888 address
for connectivity.

In the WatchGuard interface, what do I do with the Customer LAN info?
I don't really see a place to enter it in. All I see is a box for
external netblock, gateway, and ip blocks for the trusted and optional
interfaces (I already know that those are configured correctly).

Anyway, thanks so much for any help, I really appreciate it.