The "/favicon.ico" page:

Return the configured "favicon.ico" image. If no "favicon.ico" image is defined, the returned image is for the Fossil lizard icon.

The intended use case here is to supply an icon for the "fossil ui" command. For a permanent website, the recommended process is for the admin to set up a project-specific icon and reference that icon in the HTML header using a line like:

<link rel="icon" href="URL-FOR-YOUR-ICON" type="MIMETYPE"/>