Since switching to Debian stable, I’ve been using the Flatpak version of Firefox to use the latest version, which has a font rendering issue with some sites such as the web client for WhatsApp, some web mail clients, etc; showing pixelated fonts.

To fix it, edit ~/.var/app/org.mozilla.firefox/config/fontconfig/fonts.conf and paste the following:

<?xml version='1.0'?>
<!DOCTYPE fontconfig SYSTEM 'fonts.dtd'>
<fontconfig>
	<!-- Disable bitmap fonts. -->
	<selectfont>
		<rejectfont>
			<pattern>
				<patelt name="scalable">
					<bool>false</bool>
				</patelt>
			</pattern>
		</rejectfont>
	</selectfont>
</fontconfig>