Your public fixed IP banned on a few sites, or you don’t want your real IP to be seen on them ? Normally by using a proxy you can bypass this problem.
But, by adding a proxy in your IE or Firefox configuration you direct all your HTTP/HTTPS/FTP/SOCKS traffic through the proxy, something you don’t need. Remember, there are only a few websites that need this. ( Proxies are slower ).
Here is a fix for Internet Explore and Firefox ( under Windows ). We need to create a PAC file (Proxy Auto Configuration) and then instruct IE / Firefox to use it.