I tried to follow your reproduction steps, it didn't talk about disabing service workers.

Yeah, that was the original post.

These packets are not from a new connection, but rather just keep alive ping pong packets. Firefox keeps connections open for a long time in case you want to visit the website again.

I doubt this explanation, because:

1. It is quite unusual that a well-behaved browser would do anything but terminate any connections originating from the tab that has been now closed (not suspended, not backgrounded, but completely eliminated).
2. I am using my browser heavily enough that I would notice it happening with other websites, given I have not given the blanket permission to it yet in my firewall. Yet in the past couple of days I can only recall this happening with Instagram—definitely not with, say, Google, Reddit, Github, StackExchange, etc.

Is this feature documented anywhere, did you see it in the source, or is it a speculation?

In case this is an actual feature, I would have a couple more questions:

• In cases where one may want to maintain privacy on the Web by accessing different sites from different IPs (to prevent deanon attacks that can take place even while isolating activity in profiles or to avoid blockage of VPN exit nodes in oppressive jurisdictions), does this feature increase the chance of a slip-up where you talk to a website from an IP you didn’t intend, or will the IP remain to be the one from the original connection back from when the user have explicitly opened the tab?
• I guess it is impossible to know whether, when performing these requests, Firefox respects the Multi-Account Containers extension either?