At first I thought it may have been DNS, but allowing inbound/outbound UDP should fix that problem. If your machine is a DNS server, you hould also allow TCP port 53 inbound as well.
Do you allow full inbound/outbound access on TCP/UDP to localhost (127.0.0.1)? Many applications require this open to work. Mozilla may be one of them.
At the end of your INPUT, OUTPUT and FORWARD chains you should add a LOG and DROP jumps as seen below.
You should then check your /var/log/messages file to see what is being blocked and then add entries to the chains to facilitate this activity.
Hope this helps.