How can I restore a website to the "Top Sites" in Firefox after accidentally removing it?

The websites blocked from the "Top Sites" are stored in the setting browser.newtabpage.blocked that can be found in about:config. However, the names of the sites are not stored in a human-readable format, so restoring a single website is difficult. The best possibility that I have found is to completely reset the setting, which restores all pages that were dismissed from the Top Sites.

How can I restore a site to its former place after I have unintentionally removed it?

First add it back:

Add a Top Site

1. Hover the mouse pointer anywhere within the Top Sites section of tiles and click on the three-dot menu that appears in the upper right corner.

2. Click Add Top Site.

   

3. Type in a title and URL for the site, then click Add.

   A tile with the first letter from the title will appear in your Top Sites.

Then put it back in the correct place:

Rearrange Top Sites

Click and hold on a tile and drag it to another spot.

Source Customize the New Tab page