>[оверквотинг удален]
> http_access allow localnet
> заменить на
> http_access allow my_network
> Строку
> http_port 3128 intercept
> заменить на
> http_port 127.0.0.1:3128 intercept
> Убрать строку
> http_port 3129
> Выхлоп ipfw show покажите.

Загрузил правила, согласно вашим рекомендациям. Выхлоп:

00050  702 475601 divert 8668 ip from any to any via bge1
00100    0      0 allow ip from any to any via lo0
00200    0      0 deny ip from any to 127.0.0.0/8
00300    0      0 deny ip from 127.0.0.0/8 to any
00400    0      0 fwd 127.0.0.1,3128 tcp from 192.168.50.128 to any dst-port 80 in recv bge0
00500 1474 957392 allow ip from any to any
65535 9194 553357 allow ip from any to any

Также поправил конфиг сквида, но строку http_port 3129 если коментирую то сквид не ругается:
ERROR: No forward-proxy ports configured.
Согласно информации з гугла, для прозрачного прокси нужен еще один порт.

Вот конфиг сквида новый:
acl localhost src 127.0.0.1/32
acl my_network src 192.168.50.0/24

acl SSL_ports port 443
acl Safe_ports port 80          # http
acl Safe_ports port 21          # ftp
acl Safe_ports port 443         # https
acl Safe_ports port 70          # gopher
acl Safe_ports port 210         # wais
acl Safe_ports port 1025-65535  # unregistered ports
acl Safe_ports port 280         # http-mgmt
acl Safe_ports port 488         # gss-http
acl Safe_ports port 591         # filemaker
acl Safe_ports port 777         # multiling http
acl CONNECT method CONNECT

http_access allow my_network

http_access deny !Safe_ports
http_access deny CONNECT !SSL_ports
http_access allow localhost manager
http_access deny manager

http_access allow localhost

http_access deny all

http_port 127.0.0.1:3128 intercept
http_port 3129

cache_dir ufs /cache 20480 16 256
coredump_dir /cache

refresh_pattern ^ftp:         &n... 1440    20%     10080
refresh_pattern ^gopher:        1440    0%      1440
refresh_pattern -i (/cgi-bin/|\?) 0     0%      0
refresh_pattern .               0       20%     4320

Лог сквида:
2013/08/13 12:14:00 kid1| Logfile: opening log daemon:/var/log/squid/access.log
2013/08/13 12:14:00 kid1| Logfile Daemon: opening log /var/log/squid/access.log
2013/08/13 12:14:00 kid1| WARNING: no_suid: setuid(0): (1) Operation not permitted
2013/08/13 12:14:00 kid1| Squid plugin modules loaded: 0
2013/08/13 12:14:00 kid1| Adaptation support is off.
2013/08/13 12:14:00 kid1| Store logging disabled
2013/08/13 12:14:00 kid1| DNS Socket created at 0.0.0.0, FD 9
2013/08/13 12:14:00 kid1| Adding domain csia.net from /etc/resolv.conf
2013/08/13 12:14:00 kid1| Adding nameserver 192.168.50.4 from /etc/resolv.conf
2013/08/13 12:14:00 kid1| Adding nameserver 192.168.50.6 from /etc/resolv.conf
2013/08/13 12:14:00 kid1| HTCP Disabled.
2013/08/13 12:14:00 kid1| WARNING: no_suid: setuid(0): (1) Operation not permitted
2013/08/13 12:14:00 kid1| Pinger socket opened on FD 15
2013/08/13 12:14:00| pinger: Initialising ICMP pinger ...
2013/08/13 12:14:00| pinger: ICMP socket opened.
2013/08/13 12:14:00 kid1| Loaded Icons.
2013/08/13 12:14:00 kid1| Accepting NAT intercepted HTTP Socket connections at local=127.0.0.1:3128 remote=[::] FD 11 flags=41
2013/08/13 12:14:00 kid1| Accepting HTTP Socket connections at local=0.0.0.0:3129 remote=[::] FD 12 flags=9

Также включал дебаг по этому WARNING: no_suid: setuid(0): (1) Operation not permitted :

tools.cc(755) enter_suid: enter_suid: PID 70354 taking root privileges
2013/08/12 12:32:25.779| tools.cc(758) enter_suid: enter_suid: setresuid failed: (1) Operation not permitted

Результат к сожалению тот же.