Where in settings to designate the default save for bookmarks?

There isn't a setting for this anymore, and you're right about that coded value: it's the folder's internal ID, not a name, so typing a folder name in there won't work. What I do is press Ctrl+D on a page, pick the folder I want in the dialog, and Firefox keeps using that folder for the next bookmarks I save. So just save one bookmark into the folder you want and it becomes the default from then on.