Best Proxy Switcher was developed by a team who has provided proxy services since 2004. You can use Best Proxy Switcher with your own proxy servers or let it download most recently checked list from their website automatically for free.
Here is a little more about this add-on from the developers:
For advanced users and web masters who needs more proxy servers, there is an option to register at our web site for 9.99 USD and get unlimited access to our database (please use ‘register’ button within add-on to get special price). Proxy-List.org web site offers more than 10.000 working proxy servers per day during 10 years! Our high quality service is confirmed by time and our clients!
Want to learn more? You can pick up Best Proxy Switcher from the Firefox Add-ons website.