ProxySel

The Proxy selection and processing utility is a Mozilla extension that let you select a proxy from a drop down menu. You may import proxy lists in different formats, available from http://www.checkedproxylists.com/ or http://proxy-list.org/en/ and export it back to any location on disk.

I think, http://proxy-list.org/en/ is more up to date, just download the Full List. The normal way to do is to load a file from any webpage suggested above, save it on disk, and import this file into Firefox with ProxySel, test the Proxies and rate it.

Requirements:

Windows XP/Vista, Mozilla Firefox 1.0 - 2.0