Thank you for your feedback! You should have been redirected to the add-on settings page automatically when you installed the add-on.
It did not seem to be the case, however you can access the add-on settings by going to the Firefox add-ons manager (accessible via the menu "Tools" then "Add-ons" or by using Ctrl+Shift+A).
Then simply click on the "Options" button next to Default Bookmark Folder to get to the settings page.
Once there, you can set up the add-on to fit your needs!

If you have any other questions or problems, please feel free to create a new issue on GitHub (https://github.com/teddy-gustiaux/default-bookmark-folder/issues).
If you do not have an account, you can create one for free.
The Mozilla Add-Ons website only allows me to respond once to a review, so it is not the ideal place for a conversation. :)