On Linux and MacOS is quite normal to see the standard output in the shell when running Firefox from the command prompt.
On Windows, this isn’t that easy, as the “-console” command line switch creates a new window that quickly disappears when Firefox shuts down. Luckily, there’s an interesting option that seems to produce the desired behaviour on Windows: “-attach-console“.
As reported on MDN, “[…] use -attach-console to use the existing console.”.