You can edit an existing tile and modify its title and URL and possibly add an icon.

• 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

This information is of no help.

The available space is not fully used. The display is set to 4 rows, 1.5 rows are not occupied.

Additional links should be added, not existing ones changed.

The add tile is not displayed. So the question is: How do I get it displayed again, or is there an alternative option to create additional links.

Problem solved.

The pinned shortcuts can be found in the config file (about:config) in the browser.newtabpage.pinned section:

[{pinned data},{pinned data}, ... ]

The value for the tile to add new shortcuts is "null".

This value was at the beginning of the entry: [null,{pinned data},...]

Comparison with a slightly older Firefox version has shown that this value only works between individual entries. After adjusting to: [{pinned data},null,{pinned data}, ... ] the tile is displayed again and can be used to add new shortcuts.

I cannot understand why and how this value was moved to an invalid location. The problem is now solved, however.

I can only get the add "+" button if there isn't enough history to fill all available tiles. So if there is enough history available and all tiles are filled then the only way to add a tile is to edit one of the automatically filled tiles and pin a website to this tile.