Last time I checked the security of FireFox+ProxyChains+Tor (on the host system) by ip-check.c0m and it says OK, but some DNS test site says my DNS is leaking, why is that? Does this effect to Whonix?

Which DNS test site?

I am not sure I understand the question right. Not using Tor Browser, i.e. not using Firefox is discouraged because Firefox isn’t optimized for anonymity and privacy. (And no, you can not fix that with installing add-ons alone.) (See: WebBrowsers · Wiki · Legacy / Trac · GitLab)

When Firefox leaks on the host, this wouldn’t mean anything for applications running inside Whonix.

proxychains firefox dnsleak.com

It can tell which DNS I’m using. I am in the censored country, why my country allows me to visit all the censored websties?

Does DNS leaks cause IP leaks?