This is not the first time this issue is mentioned. And still, it is a Firefox issue. Native Messaging launches a child process from the browser, and for some reason Firefox also copies the socket handles to the child process. Because of this the proxy shows connections to locations it doesn't have an access to. Some of the handles can be connections that are already terminated. The only thing the proxy does is listening for stdin and connecting to a local Unix socket by KeePassXC. Anyone can take a look at the source code and verify this, or compile a build manually where the problem still exists.