Sorry for being so long to understand :(
Polylang first compares this code to the browser preferences. A match is found if the browser preference starts by the language code 'cn'. In your case it can't happen as this code does not exist.
Then there is a second try to compare the locale (zh-CN) with the browser preferences. So it won't work if only 'zh' is in browser preference but should work if there is 'zh-CN'. The issue is that this comparison is case sensitive and:
Chrome sends zh-CN
while Firefox sends zh-cn :(
So please try modifying the file frontend/choose-lang.php at line 115 from
if (empty($pref_lang) && (0 === stripos($accept_lang, $language->slug) || $accept_lang == str_replace('_', '-', $language->locale)) )to
if (empty($pref_lang) && (0 === stripos($accept_lang, $language->slug) || 0 === strcmp($accept_lang, str_replace('_', '-', $language->locale))) )