why can't I find my imported addressbook as a list from an old e-mailservice

I think you need to ask your tech person exactly what they imported to where. Thunderbird has common address books across accounts, so if you have a GMail and a Comcast account, a single address book is used for both. Delete one and add a Telefonica account and the address books remain unchanged. I am guessing the imported the address book to your new providers web mail, but that is just one of many things they could have done.