Your proxy doesn't properly support renegotiation handshakes. This is probably just the tip of the proverbial iceberg, leading to stalled connections. Does it even support http/2? Does it have a usable/stable SPDY implementation?
For important reasons, you should not use an https filtering proxy, ever. You are MitM-ing yourself and preventing proper TLS authentication. In addition, you may be severely weakening your outbound connection security.
I suggest you don't use the filtering proxy with Pale Moon.
EDIT: Seems not closing connections is a known issue in that proxy. You'll run out of available server connections very quickly, that way. If connection pools get saturated, no new connections can be made. So.. that's a serious issue with the proxy. Firefox may have less of an immediate issue because Firefox allows by default for a very large number of concurrent connections.
A potential workaround would be to increase network.http.max-connections and network.http.max-persistent-connections-per-proxy (try doubling the amount)