Anonymous proxy not working only for Firefox. For Chrome - OK

问题

I tried to configure anonymous proxy (centos+squid), but some sites still shows me my external IP (not proxy IP) for Firefox only.

So, squid configuration as follows:

server_persistent_connections off
forwarded_for off

request_header_access From deny all
request_header_access Referer deny all
request_header_access Proxy-Connection deny all
request_header_access Server deny all
request_header_access Via deny all
request_header_access WWW-Authenticate deny all
request_header_access X-Forwarded-For deny all

For Chrome it works as a charm: my external IP address replaced to proxy IP but some sneaky sites like those

https://www.privateinternetaccess.com/pages/whats-my-ip/

https://www.whatismybrowser.com/

are able to detect my external IP only when I am using Firefox (for Chrome - OK, they showing proxy IP)

Some not so sophisticated like http://www.whatismyip.com/ shows me proxy IP for Firefox and Chrome

Question: how https://www.privateinternetaccess.com/pages/whats-my-ip/ and https://www.whatismybrowser.com/ discovers my IP when I am using Firefox? What am I miss ? Is it somehow connected with SSL (note, they both using HTTPS, while http://www.whatismyip.com/ using HTTP) ?

Update: disabling webRTC in FF did not help

Thanks