I read the MDN documentation about how to get a reference to the bookmarks tree und to navigate within. But how do I know which folder represents the bookmarks toolbar? This toolbar is named differently depending on the locale. I am not sure whether a user is still allowed to select an arbitrary folder for this, but this was possible some years ago.
I found this post: Programmatically bookmark pages in bookmark toolbar using firefox extension
but this is not about the new webextension API that everybody has to use now.