Finally got it to work.
What I did?
1. Re installed everything again. I noted that I made a mistake with the config.yml and my-passwords.yml files. On my first installation I saved them in home ~ instead of root / and installed OpenEdX on root / . So for the second installation I saved them in root / and installed also in root.
After that I noticed that I was able to sing-in from firefox with the changes suggested by @Agrendalath.
2. I found this suggesting to change
DCS_SESSION_COOKIE_SAMESITE = 'None'
to
DCS_SESSION_COOKIE_SAMESITE = 'lax'
in common.py ( /edx/app/edxapp/edx-platform/lms/envs ). So I did and worked.
That thread also suggested to add a line in /edx/etc/lms.yml with:
DCS_SESSION_COOKIE_SAMESITE: lax
But I’m not sure if that’s helpful or needed so I didn’t. Do you think that I must add that line in lms.yml?
Thanks in advance,