HT1810: Mac OS X v10.5, 10.6: About the Application FirewallLearn about Mac OS X v10.5, 10.6: About the Application Firewall
Your question doesn't make much sense.
There is little or no relationship between WEP encryption on your wireless network and your machine's firewall.
Encryption on your wireless link protects your network against snoopers who are listening in - the stronger the encryption, the harder it is for them to intercept your traffic and see what's going on. Strong encryption also makes it harder for rogue users to get onto your network and piggy-back on your internet connection.
Nowadays, WEP is considered insufficient since it's easy enough for a skilled hacker to break the access keys with just a few minutes work. It's better than nothing, but not something you should be happy to run.
The firewall, on the other hand, doesn't care about the kind of network connection you have (wired, wireless, etc.), and only serves to prevent unwanted traffic hitting your system. However, the firewall has no easy way to differentiate between legitimate and illigitimate users on your local network - they all look the same, and it does nothing to prevent access to services you've enabled.
If you're running an insecure wireless network (which you are) then it's easy enough for someone to jump on your wireless network. If you've enabled any kind of service on your machine (e.g. remote login, web service, file sharing, etc.) then that user has unfettered access to that service whether or not your firewall is running.
If that all concerns you, consider a wired connection to your router, or get a separate wireless bridge that can give you WPA2 encryption and let you turn off the wireless link in the FiOS router.