u/ConProg

Easy way, but may be removed in a future version of Firefox:

• Go to about:config and set "browser.newtabpage.activity-stream.nova.enabled" to false.

Alternative way, use CSS code:

1. Open your profile folder: Help -> More Troubleshooting Information -> Profile Folder -> Open Folder

2. Create a folder named "chrome" if it doesn't exist. Create a "userContent.css" file in this folder if it doesn't exist.

3. Add this CSS code to userContent.css and then close and restart Firefox:

@-moz-document url(about:newtab), url(about:home) {


/* Restores pre-Nova tile size and spacing */
.nova-enabled .top-sites-list {
  column-gap: 0 !important;
}
.nova-enabled .top-site-outer {
  width: 120px !important;
}
.top-site-outer {
  margin-block-end: var(--space-medium) !important;
}


/* Removes Nova tile mouseover enlarge animation */
.nova-enabled .top-site-outer:is(:hover, :focus-within, .active) .tile {
  width: 64px !important;
  height: 64px !important;
}


/* Restores pre-Nova tile mouseover gray background color */
.nova-enabled .top-sites-list:not(.dnd-active) .top-site-outer:is(.active, :focus, :hover) {
  background: color-mix(in srgb, var(--button-text-color-hover) 14%, transparent) !important;
}


/* Removes Nova animation for pin and ... icons */
.nova-enabled .top-site-outer:is(:hover, :focus-within, .active) .icon-pin-small,
.nova-enabled .top-site-outer:is(:hover, :focus-within, .active) .context-menu-button {
  transition: none !important;
}


}

Thanks to ResurgamS13 for one of the tweaks.