[CentOS] https question

Hi

I have question about https

I am using mozila to access gmail as https://mail.google.com/mail

Why mozilla prompts me the alert box?

"You have requested an encrypted page that contains some unencrypted information. Information that you see or enter on this page could easily be read by a third party."

How mozilla knows I have data not encrypted? 

ls https secured?

Thank you

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