I’ve been getting a network error whenever I open a GitHub codespace:

Oh no, it looks like you are offline!

If you verified that your connection is fine, your firewall might be blocking the connection. Please refer to this troubleshooting guide to diagnose the issue.

Very annoying, but I figured out that it was a FireFox (maybe just LibreWolf?) specific issue with the “enhanced tracking protection” feature.
Disabled that and everything works fine.