If you do not need to use a proxy to connect to internet then try to select "No Proxy" if "Use the system proxy settings" or one of the others do not work properly. See "Firefox connection settings":

Configure Firefox to Use a Proxy Server - Help Guides Configure Firefox to Use a Proxy Server Note: We are using Firefox 35.0.1 to demonstrate the changes, but these steps should be similar for all versions of Firefox. Open the Firefox menu in the top right corner and click on ' Options' . How to Set Proxy Server in Chrome, Firefox, Edge, Opera How to Setup a Proxy Server in Chrome, Firefox, Safari, Opera and other Popular Browsers. As already explained above, the main focus of today’s guide is to show you the step-to-step guide on how to configure/setup proxy server addresses on Chrome, Firefox, Opera and other popular web browsers.. It covers the manual proxy setup instruction for the listed internet browsers. Proxy | NordVPN Customer Support / Proxy; Proxy. Tutorials on how to set up proxy with NordVPN. Proxy setup on uTorrent; Proxy setup on qBittorrent; Proxy setup on BitTorrent; Proxy setup on Chrome; Proxy setup on Firefox; Proxy setup on Deluge; Proxy setup on Vuze; Proxy setup on Safari; Proxy setup on Opera; Proxy setup on … Turn Off Proxy Server in Firefox – Xfinity®

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Scroll down to see “Network Proxy” and adjust the settings slightly to fix the problem. Basically, the role of a proxy server is to act as a connecting link between your system and the remote server. It is possible that the remote server has refused to identify the proxy server (which could be a VPN) in your browser.

The proxy server is refusing connections | Firefox Support

Setting up a proxy server in Firefox. If you want to connect to Firefox with a proxy, then the good news is, Firefox is a highly flexible browser that allows you to do just about anything in it. Sure, it has some trade offs but browsing with a proxy is one of the most secure ways to browse the internet. How To Check the Proxy Server Settings on Your Computer Oct 02, 2017 Set an HTTP proxy server in Firefox - TrishTech.com Jul 10, 2010 Why People Use Proxy Server and How to Use Proxy Server