Sure you could see them, after all you can do a ps and see all processes, but shouldn’t the IPC between firejails be forbidden? How is firejail 2 talking to firejail 1?
Which you can prevent by mounting /proc with the "hidepid" option.
I think it depends on what kind of IPC. I'm not familiar enough with the details of how Firefox does it to know how that would interact with firejails.