security questions

Hi I have questions about security 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." 1/ Can network software help to check? if yes. which software and how? 2/ How mozilla knows I have data not encrypted? 3/ ls https secured? If not. why it is PCI? Thank you Send instant messages to your online friends http://uk.messenger.yahoo.com