- Which OS do you use?
- Which version of Firefox do you have?
- What is the exact URL where you are trying to login to?
- What happens when you "cannot even use it to log in", do you get an error?
- Is the checkbox under Settings > Password > Remember always checked?

Update on 26 June 2019:
Thanks for the extra info.
The forum is responsible for the connection between an existing user and the SQRL identity. I cannot do anything in that process. Feel free to post a message on that forum with this problem.

I cannot fit a text longer than 4 characters on the popup icon so i chose "Auth". Please let me know if you have a better suggestion.