Small breakthrough: I booted the system without problems to tty1 (I believe this is called single user mode), logged in as an old user and now I can see all my data, logged in as old me.
Nice.
Do you still recommend to backup from live usb and upgrade from there?
I would backup from live usb and then when that is done stop using the live usb, reboot and try to upgrade via the recovery mode.
Waterfox has had some bad press. I don't remember details but here's something to read :
I prefer to stick to the no nonsense LibreWolf and when some things don't work fall back to Firefox :