Looking on line there is something called BT Mail and there is something else called BT Yahoo Mail, so you'll need to tell us which you are using if you want detailed guidance. The general principle, though, is to export your data from BT and then import that into Thunderbird. As a first step look in the BT help menu to find what format it uses to export information. One common format is 'comma separated values' or CSV, and Thunderbird can cope with that among others.

Thank you for your help - I am nearly there but need to sort out how to list my address book in Thunderbird. I think that I can do this when I have a little more time.

I have now established that the BT Contacts are in a CSV file and I have imported that to Thunderbird but the listing headers do not match and it is difficult to locate an email address for known persons. Also when writing a new message and typing the "To" the listing does not come up. I am using BT Mail and any further help will be appreciated.