For some reason you do not want people to know your IP address ( which is unique at the time), and you would like to keep yourself anonymous for some reason.

Firefox Extension -TOR - A firefox extension to allow you easy to switch to any proxy server. You can preset your favorite proxy servers. This is very useful if a website have limited visitor connection for certain countries or geographical regions. (For example: Some Hong Kong newspaper website only accept Hong Kong’s IP address)

Hide IP Platinum - is a automated tools to find you proxy server available to you and set it as your proxy server automatically. But this is not a freeware, you can try download the trial version and try.