What is it that is not working and how can you tell?
Naturally everyone can access the web server if you just turn forwarding on for everyone. Since you are running a firewall, I guess there are some people or some kinds of access you want to block.
Instead of direct IP forwarding, you may want to try IP masquerade - normally used at gatways - or try setting your firewall box as a bridge or transparent proxy.
I wouldn't normally use both pre and post-routing. example of a gateway firewall:
http://www.linuxquestions.org/questi...0/#post2224036
... and an example with masquerade:
http://www.linuxquestions.org/questi...2/#post2790552
... as a bridge:
http://www.linuxjournal.com/article/8172
... transparent proxy:
http://www.faqs.org/docs/Linux-mini/...rentProxy.html
... now, typical free software, you have too many choices.
Happy hacking.