First, I'm going to make an assumption: What you call a 'firewall' is really a 4-port broadband router, correct?
Out of the box, those routers are set to use 192.168.1.1 (usually) as their IP addy, and DHCP is enabled. That setup should work fine in 99.999% of cases. If you want to change it, you can change it to anything you want.
Only have 1 device with DHCP enabled on any individual network segment, so if 2 have it enabled, disable one of them.
I never use 0 as the last octet, but I suppose you could.
If you want the addy's to be 10.0.0.x and have it manually configured, then pick a number for each device - i.e., router = 10.0.0.0, PC1 = 10.0.0.1, PC2 = 10.0.0.2, etc. Make sure every device stays in the 10.0.0.x 'subnet', and make your subnet mask 255.255.255.0. Default gateway will be the IP addy of the router. Modem shouldn't have a specific addy, because most ADSL modems are really just bridges and htey don't care.
<font color=blue>Outside of a dog, a book is man's best friend. Inside the dog its too dark to read.
-- Groucho Marx</font color=blue>
By default, your router will be a 192.168.x.x number. Change that to something like 10.0.0.1 (routers generally are .1).
Subnet mask can technically be whatever in this situation.. since you want to use the standard corporation IP address range: Class A, subnet will be 255.0.0.0
So, you can do 10.0.0.1 with subnet of 255.0.0.0 - or 255.255.255.0 - you're not subnetting or supernetting, so it really doesn't matter. Just stick to 255.255.255.0 for standard troubleshooting.
Assign your computers whatever, 10.0.0.100/101/102/....
In most cases, you'd setup DHCP to dish this stuff out and be done with it, but if you don't want to use DHCP, disable it.
Default gateway will be 10.0.0.1 (router's IP address)
The only IP that will be different is the external (Public) IP address of your Router.