You can sort individual folders. For example, if you open the Bookmarks Sidebar (Ctrl+B), then right-click a folder, such as Bookmarks Menu, you should find a "Sort by Name" item. That will sort the main level of the Bookmarks Menu, but it may not sort subfolders. If you want a tool that can sort everything at once, or re-sort when you add a new bookmark, you could use an add-on. For example, here's one I tested years ago, but it looks like it probably still works: https://addons.mozilla.org/firefox/addon/sort-bookmarks-webext/