[nitrogen]

With Firefox, opening a new tab in a non default container has a terrible default shortcut, and you can’t remap it to anything sensible (like cmd + some letter).

Just out of curiosity, have you tried creating a shortcut based on the menu name in the macOS global settings? As a Linux user who sometimes works for companies that issue Macs, that functionality goes a long way to preserving my sanity, but sometimes apps just hard-code a shortcut instead of using the OS.