This is not the first time the issues rises its head from the depths.
Here's the KeePassXC-Browser issue: https://github.com/keepassxreboot/keepassxc-browser/issues/100
And here's the Mozilla one: https://bugzilla.mozilla.org/show_bug.cgi?id=1463873

Again: this is a Firefox bug. If you look the connections keepassxc-proxy makes with Chromium-based browsers, you'll see none.

And to clarify: keepassxc-proxy does not make any connections. These are file descriptors that are leaking from the parent process (Firefox).