This article is 3 years old - https://superuser.com/questions/857181/firefox-is-it-possible-to-change-the-number-of-recently-used-bookmark-folders - for ~ Firefox 34. This is a Legacy extension - https://addons.mozilla.org/ja/firefox/addon/change-bookmark-recent-folder-/ - not compatible with Firefox 57-up which uses WebExtensions. Sorry, I am not aware of a WebExtension that had that type of feature. Maybe someone knows of one.