I'm seeing this in the logging in a number of places: "(generic/ERR) NrTcpSocket::OnClose 23743e54c00 reason=2152400251 name=NS_ERROR_PROXY_FORBIDDEN " The proxy you're trying to use is probably responding with a 403. You could probably verify this using wireshark. https://searchfox.org/mozilla-central/rev/1c391443f770eddc7cde9e52dba5ef50cc233c06/netwerk/protocol/http/nsHttp.cpp#889
Bug 1770127 Comment 4 Edit History
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I'm seeing this in the logging in a number of places: "(generic/ERR) NrTcpSocket::OnClose 23743e54c00 reason=2152400251 name=NS_ERROR_PROXY_FORBIDDEN " The proxy you're trying to use is probably responding with a 403. You could verify this using wireshark if the proxy is not using SSL, I suppose. https://searchfox.org/mozilla-central/rev/1c391443f770eddc7cde9e52dba5ef50cc233c06/netwerk/protocol/http/nsHttp.cpp#889