How to edit private proxy settings in Firefox
Go to Tools > Options
Click on Network > Settings
In the Connection Settings window, click on Manual proxy configuration.
Enter the IP in HTTP Proxy and SSL Proxy field.
Enter 3128 or 8800 in Port field.
This is how your proxy settings should look like.
Click Ok, then try to browse.
Becomes some ISP block ports 3128, 8800, you can also try port 52931 instead.