Normally, new bookmarks are saved in the "Other Bookmarks" category. Is yours saving somewhere else?

If you don't get the drop-down to select your preferred folder, click a star icon after it turns blue and turn on the editor:

If you are hiding the star icon, you also can change the setting this way (the editor dialog will drop from the Page Actions menu button •••):

(1) In a new tab, type or paste about:config in the address bar and press Enter/Return. Click the button accepting the risk.

(2) In the search box above the list, type or paste book and pause while the list is filtered

(3) If the browser.bookmarks.editDialog.showForNewBookmarks preference is bolded and has a value of false, double-click it to restore the default value of true