It might be easiest to use the Bookmarks Sidebar for this. You can open that using the menu or by pressing Ctrl+b.

Right-click "Bookmarks Menu" and choose Sort by Name. (Screen shot example attached) This will alphabetize the bookmarks on the main level of the menu, but if I recall correctly, it doesn't alphabetize the bookmarks in individual folders. To sort individual folders, right-click the name of the folder and choose Sort by Name.

Does that work?

If you have a lot of folders, this could take a while, especially if you want to do this often. In that case, it would be worth checking the Add-ons site to see whether there is an add-on for this. https://addons.mozilla.org/firefox/

Note that you can only right-click a folder to sort this folder.

You can also do this in the Bookmarks Manager (Library) via the right-click context menu.

Do not use the Views menu in the Bookmarks Manager because that is only for viewing purposes and is the same as clicking a column header to sort by this column.

The sort by name function is not working at all.