Firefox is a web browser and does neither handle your emails nor your address book. Since you were accessing your emails and address book through webmail, you'd need to contact your email provider for help exporting the address book to a format Thunderbird can import. The preferred format would be LDIF (.ldif). Less prefered (because more complicated) is comma-separated (.csv). In case of Gmail Thunderbird can sync with the Gmail address book directly by installing an add-on. https://addons.mozilla.org/en-US/thunderbird/addon/gcontactsync/

Hello christ1. I got Thunderbird installed, and then downloaded: https://addons.mozilla.org/en-US/thunderbird/addon/gcontactsync/ as you suggested. When i try to retrieve mail, i get a message that Thunderbird is "unable to locate mail spool file" Can you help again? Have not got to syncing yet, don't know how to do it. Thanks, joy@cardboard.net or joyhill9995@gmail.com

When i try to retrieve mail, i get a message that Thunderbird is "unable to locate mail spool file"

When you set up your (Gmail?) account, run the New Account wizard and let Thunderbird detect the correct server settings. https://support.mozilla.org/en-US/kb/automatic-account-configuration

Have not got to syncing yet, don't know how to do it.

The gContactSync download site has a link to a FAQ. Please read the FAQ. There is also a link to a support forum.