Connecting internet applications

That's an unusual problem unless you tampered with Firefox's proxy settings. Try checking the proxy settings to see if anything other than the "use no proxy" or whatever is checked.

click edit > options > advanced > network > settings

and you should see the proxy settings from there

The first thing is to make sure that you are really connected to the net. Try pinging Google. Then, just to be sure, you can use "echo -e 'GET / HTTP/1.0\n\n' | ncat www.google.com 80".

If that is working fine, you could try a packet sniffer like Wireshark to see where the information was trying to be sent. It could be a setting in your programs, but as it effects multiple programs I would think it is a system wide setting. Some things to check are your routing table and your firewall settings. It seems strange that you can connect to the internet, but applications can not. You may want to try some different applications, perhaps Lynx, to see if they are effected too.

Pop a shell and try /sbin/ifconfig -a. Look for your wifi device (usually wlan0) and make sure you're getting an IP assigned. It will be around the second line for the relevant device and labeled by "inet addr". If you can't find this, then you haven't had an IP address assigned. Try dhclient wlan0 or dhcpcd wlan0 or whatever DHCP client to request one.

Try ping google.com. If it says "unknown host google.com" try host google.com. If these also fail then either your DNS server is set wrong or there's serious weirdness with your routing. Usually, DHCP should assign a DNS server automatically so it shouldn't be a problem.

Finally, try /sbin/route. There are two columns here we care about labeled Destination and Gateway. Find the row with a "default" Destination and get the matching Gateway. Try to ping this IP. (ping 192.168.0.1, for example)

Also check sudo iptables -L. It should look something like:

Chain INPUT (policy ACCEPT)
target prot opt source destination

Chain FORWARD (policy ACCEPT)
target prot opt source destination

Chain OUTPUT (policy ACCEPT)
target prot opt source destination

If not, say:
sudo iptables -P INPUT ACCEPT
sudo iptables -P OUTPUT ACCEPT
sudo iptables -F INPUT
sudo iptables -F OUTPUT

There are dozens of things that could be wrong, the results of these should point out the most common problems though.