After changing the proxy server in the browser, some sites see my real IP, while others that I registered. Why?

Question

Computer networks

Actually, the question is above. Or advise, please, other solutions for changing ip. mozilla browser.

Answer 1

Some proxies pass your real address in a header like x-forwarded-for.
There are also several other ways to determine your real IP.
Check out the site https://whoer.net/ru (extended version) - everything that can be found out about you through the browser is explored there.