To avoid proxy authentication poup in firefox just follow these steps:
1. In the url address field in Firefox, enter about:config.
2. Click the I'll be careful, I promise! button.
3. In the Filter field, enter network.negotiate-auth.allow-proxies.
4. Right click network.negotiate-auth.allow-proxies and select Toggle. This changes the value false to true and your are good to go to avoid the proxy authentication popup. popup will not show again.
enjoy!