Iptables – Squid as Transparent Proxy Opens only https websites not normal http

I installed and configured squid proxy as transparent proxy server. when i tried using firefox the proxy server works fine and i can access both ports https and http. but when i transparnet, i can access https websites only like https://google.com not http://google.com

Here is my squid.conf look like:

acl our_networks src 192.168.20.0/24
http_access allow our_networks
#Recommended minimum configuration:
acl all src all
acl manager proto cache_object
acl localhost src 127.0.0.1/32
acl to_localhost dst 127.0.0.0/8 0.0.0.0/32
#
# Example rule allowing access from your local networks.
# Adapt to list your (internal) IP networks from where browsing
# should be allowed
acl localnet src 10.0.0.0/8     # RFC1918 possible internal network
acl localnet src 172.16.0.0/12  # RFC1918 possible internal network
acl localnet src 192.168.0.0/16 # RFC1918 possible internal network
#
acl SSL_ports port 443          # https
acl SSL_ports port 563          # snews
acl SSL_ports port 873          # rsync
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 Safe_ports port 631         # cups
acl Safe_ports port 873         # rsync
acl Safe_ports port 901         # SWAT
acl purge method PURGE
acl CONNECT method CONNECT


http_port 3128 transparent
icp_port 0

And here is my IPtable config

$IPTABLES -t nat -A PREROUTING -i eth1 -p tcp -m tcp --dport 80 -j DNAT --to-destination 192.168.20.21:3128
#$IPTABLES -t nat -A PREROUTING -i eth0 -p tcp -m tcp --dport 80 -j REDIRECT --to-ports 3128

What did I miss here? I'm running squid 2.7 stable 9

Thanks,