#firstHeading should be right. However, $wgJSBreadCrumbsCSSSelectorHorizontal doesn't work as advertised (and the whole logic of choosing the right selector programmatically seems to be somewhat flawed), so i ended up monkeypatching th whole thing to make it work:
# diff -ub resources/JSBreadCrumbs.js{.orig,}
--- resources/JSBreadCrumbs.js.orig 2021-04-14 12:07:31.194729971 +0200
+++ resources/JSBreadCrumbs.js 2021-04-14 12:09:01.268802485 +0200
@@ -103,7 +103,8 @@
} else if ( skin === 'foreground' ) {
selector = '#mw-js-message';
} else {
- selector = '#top';
+ // selector = '#top';
+ selector = '#firstHeading';
}
$( selector ).before(