Real world IP address of a machine behind a firewall ?

[Shaul Karl]

I am behind a firewall and/or Proxy server (I still have to learn what is the 
difference between those 2 and when to use each).
1) How can I find out the IP address that the Proxy server shows to other 
hosts on the Internet when I am connecting to them ? Is this address fixed ?
2) Does any one can comment on using yi.org with a situation like mine ?
3) Is there a specific HOWTO/FAQ etc about things you can do and how to do it 
when you are behind firewall and/or Proxy server ?