Below are the logs captured on Firewall .The proxy black box which is running proxy service for all internal LAN users .If you are saying this is a TCP SYN,ACK request then Firewall shoudl have allowed these requestes ae per stateful functionality and as per my understanding syn ack request should not be on same port(may be am worng ) .but here the out side websites are querying to proxy server as source port 80 and destination random port.I wounder why would any one try to connect proxy on port 80 as source and this is not valid source port as per IEEE ..
What Frederico has said could be the case. The logs that you seeing must be in relation to replies from web servers in response to requests from your proxy server.
That is, for example, when an inside user triews to access a website, the request goes to your proxy server which then initiates the connection on behalf of your user. For this packet, the source port will be random (>1024) and destination will TCP/80.
When the outside server replies to this request, now the source port will be changed to TCP/80 and destination port will be the random port the proxy server used initially (>1024). It is these packets that are getting dropped by the firewall.
To confirm what is going wrong, could you please post the exact set of logs you are getting on the firewall (with masked IP addresses)?