This is entirely server-side UA sniffing going wrong. You get an empty HTML doc, only a doctype, with a Firefox Android UA. You can reproduce this with curl,
$ curl -H "User-Agent: Mozilla/5.0 (Android 10; Mobile; rv:123.0) Gecko/123.0 Firefox/123.0" https://www.google.com
%
and it seems that this affects all UA strings with versions >= 65.