Index: wp-includes/general-template.php
===================================================================
--- wp-includes/general-template.php	(revision 48613)
+++ wp-includes/general-template.php	(working copy)
@@ -1017,8 +1017,13 @@
 
 	// We have a logo. Logo is go.
 	if ( $custom_logo_id ) {
+		/*
+		 * Disallow 'loading' attribute to avoid triggering an issue Firefox seems to have.
+		 * See https://core.trac.wordpress.org/ticket/50933.
+		 */
 		$custom_logo_attr = array(
 			'class' => 'custom-logo',
+			'loading' => false,
 		);
 
 		$unlink_homepage_logo = (bool) get_theme_support( 'custom-logo', 'unlink-homepage-logo' );