Google Chrome has own widget for bookmarks, but Android Firefox does not. I am thinking how to proceed to do the feature request for the widget.
OS: Oreo 8.0
Phone: Oneplus 3T
Unfortunately, Firefox doesn't have a bookmarks widget, as stated here
You can definitely send your request to them. To do that, open Firefox, go to the menu-> settings -> Mozilla Firefox -> give feedback -> choose one of the options, and you will then have an option of writing in your request.