You can change proxy in firefox easily to surf the Internet Anonymously or surf blocked Websites in your country.
save those proxies in a notepad file, name it as proxylist1.txt or any u want.
now do these steps to change proxies:
Tools -> Options -> Advanced -> Network -> Connections -> settings.
then click on manual proxy configuration.
Enter the proxy and the port from the file u just saved (proxylist1.txt) and click on Use this proxy server for all protocols.
click on OK.
go to this site and check your new IP address
For best results, always use fresh proxies. Google for more proxies.
about the speeds: download speeds will depend upon the proxies.
Also check How To Open blocked Websites in Internet Explorer.