If all the tiles are filled by Firefox and you can't add an extra row via the cog icon, then you can hover an existing tile and click Edit in the tree-dot menu tp replace this tile with a URL of your choice.

• https://support.mozilla.org/en-US/kb/customize-items-on-firefox-new-tab-page
• https://support.mozilla.org/en-US/kb/customize-your-new-tab-page

Hello,

 Thank you for your reply.  I would like to add new shortcuts, not change the shortcuts I already have and I do not use Firefox-filled or sponsored shortcuts.  

I have however found a temporary workaround; upon closing and reopening the browser (as, I have it set to clear everything upon closing), I can click the cog icon on the home page then tick the checkbox for Sponsored Shortcuts and allow them to be displayed, then the plus-sign tile to add more shortcuts appears, but will disappear again as soon as you dismiss or delete any of the sponsored shortcuts you don't want. I've submitted bug reports but I guess all we can do is hope Mozilla will fix the bug in an upcoming update.