how to Set Proxy Credentials

Question

I have to create a program to set different proxies by using code. Different proxies are taken through a company who has provided address, user name and password to access proxies. I have to fetch them by code and have to apply proxy for mozilla browser. When I applied code for proxies, when i start browser, each time it asks user name and password. 

I want to set that user name and password programatically. How can i do that? Any reference, code?

Thanks in advance.