With Firefox, accessing the extension settings is a breeze.
Simply locate the AllTheBookmarks icon on your toolbar and right-click the icon.
From the dropdown menu that appears, select Manage Extension.
The extension management page should appear similar to this. In the top right, click these three dots and select Options.
You should be greeted by a settings page very similar to the one below.
(If you need help pinning the extension to your toolbar for easy access, we have a separate tutorial for that.)