I don't know of a way to sync your iPhone contacts with TB, unless they are e.g. contained in a gmail account, but you can transfer contacts to TB by first [https://www.wideanglesoftware.com/blog/export-contacts-from-iphone-to-computer.php#icloud exporting them as vCards from iCloud], and then [https://support.mozilla.org/en-US/kb/switching-thunderbird#w_importing-windows-mail-contacts importing them to TB] with the [https://freeshell.de//~kaosmos/morecols-en.html MoreFunctionsForAddressBook] add-on.