Alright, I found it: if hoxx vpn addon is either disabled or uninstalled, it changes (at least) these 2 settings:
network.proxy.proxy_over_tls to false (true is default)
network.proxy.type to 0, even if another VPN connection is active at that time.
It's the former setting that is rendering the other addons ineffective - the other addons are not touching that setting upon install, since it's boolean type and the default value is the one they need.
I find this to be a very aggressive behavior from hoxx to do this
