Have you tried these steps :

• Open Bookmark Library (CTRL+Shift+B) > Show All Bookmarks.
• Select Folder from left panel from where your both sites are stored.
• From Right panel select those two websites (select one by one or click on both url with Pressing Shift key from keyboard. )
• Now Either Use Del key or right click and select delete from menu .

This will delete selected sites from your bookmark section.

Please go through this answer (since i don't have a mac right now :) ) and revert if that helps : https://support.mozilla.org/en-US/questions/957666

Open the Bookmarks Manager. At the top is a search bar.

You can remove the bookmarks in the Bookmarks Manager (Library).

• https://support.mozilla.org/kb/how-do-i-delete-bookmark

Select the bookmark and press Shift+Delete or use the Delete item in the Organize menu or the right-click context menu.

Bookmarks stored on the Bookmarks Toolbar are located in the Bookmarks Toolbar folder.