It is very easy way to use proxy. The plug-in or add-on is called Proxysel, and can be downloaded for free here. Install it and then restart Firefox to activate the software.

You should now see a window that looks a little like this at the top of the screen, except yours will be empty, as you have not ‘loaded up’ any proxy servers yet:

Go to Checked Proxy Lists and scroll down the page until you see the area
where the various different lists are available:

The file that you need to use is the one right at the top, the ‘high
anonymous proxy list’.
Download the detailed CSV file by clicking the link in the brackets.
Now, go back to the Proxysel window at the top of your browser and left
click on the word ‘Proxysel’ itself. Next, hit ‘Import – Export Proxylist’ >
‘Import List’ > ‘and overwrite Proxylist’:
Look for the CSV file that you just downloaded and click on it in the window
to add it to Proxysel.
You should now see confirmation that all of the proxy addresses that were
contained in the CSV have been added to the program, and if you click on

the dropdown icon to the right of the Proxysel window, you should be able to
see the complete list of proxies that you just added:

To use any of the proxies listed, highlight the one that you want to activate
and click on the tiny monitor icon to the right of the Proxysel window. It
should turn light blue as seen here:

And you should also see the numerical IP address displayed at the bottom
right of your browser screen as well.

http://www.checkedproxylists.com/