facebook - extention does not work

[AlexeyAM]

Attempt to open www.facebook.com:

Browser response:

Unable to find the proxy server

Firefox is configured to use a proxy server that can’t be found.

Check the proxy settings to make sure that they are correct.
Check to make sure your computer has a working network connection.
If your computer or network is protected by a firewall or proxy, make sure that Firefox is permitted to access the web.

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linkedin.com opens normally

[ilyaigpetrov]

Confirm, the error is UNKNOWN_PROXY_HOST: