@nthn,
Now you mention it, I had looked at logins.json. It contains UN/PW in an encrypted form. Based on what you say keys4.db is the answer. If only I knew how to use it

@mrsellout,
I will have to try it in a VM. I only have a Windoze PC available (and no Firefox, only Pale Moon) at the moment and your command does not work.