Please have a look at [[Thunderbird and Gmail|this support article]] for info about setting up a Gmail account and some (IMAP) particularities that you may need to be aware of, including the links to articles at Gmail if interested. As for contacts, you should be able to share them in Thunderbird regardless of the account. Existing emails from the POP account can be copied or moved to folders under your newly created account. The same can be done with sent mails, after which they will be synchronized with Gmail. That said, this should be valid for a new Gmail IMAP account in Thunderbird, but if you have created an IMAP account with another provider, I don’t think anything is different except for the Gmail instructions. Does this help?