My Firefox userChrome.css & userContent.css

userChrome.css

/* Move Find Bar position. Based on: https://pastebin.com/sucmEx7N */
.browserContainer > findbar {
	/* Position the Find Bar above the page */
	order: -1 !important;
	/* Move Find Bar border from top to bottom */
	border-top: none !important;
	border-bottom: 1px solid hsl(240, 5%, 5%) !important;
}

userContent.css

/* ACTIVITY STREAM */
@-moz-document url(about:home), url(about:newtab)
{
	/* Round Top Sites tiles */
	.activity-stream .top-site-outer .tile, .activity-stream .top-site-outer .top-site-icon, .activity-stream .top-site-outer .letter-fallback {
		border-radius: 25% !important;
	}

	/* Fit Top Sites icons to wrapper */
	.activity-stream .top-site-outer:not(.add-button) .tile .icon-wrapper {
		width: 100% !important;
		height: 100% !important;
	}

	/* Remove shadow from placeholder tile */
	.activity-stream .top-site-outer.placeholder .tile {
		box-shadow: none !important;
	}

	/* Remove background from placeholder tile */
	.activity-stream .top-site-outer.placeholder.add-button .tile {
		background-color: transparent !important;
	}

	/* Remove title from placeholder tile */
	.activity-stream .top-site-outer.placeholder.add-button .title {
		opacity: 0;
	}

	/* Lower the Top Sites context menu to the inside of the inner container */
	.activity-stream .top-site-outer .context-menu-button {
		top: 0px !important;
	}

	/* Hide pinned icon on Top Sites */
	.activity-stream .icon.icon-pin-small {
		display: none !important;
	}

	/* Change Top Sites title weight */
	.activity-stream .title.pinned span {
		font-weight: bold;
	}
}

Nice. This fixed up my top sites. Probably should embed it within
@-moz-document url(about:newtab) { ... }
https://developer.mozilla.org/en-US/docs/Web/CSS/Mozilla_Extensions#at-rules