Hi Martin,
You can try the following steps to import MAB to Gmail:
1. Go to Thunderbird account and select Address Book (from the menu bar). It will open another window containing contacts.
2. Now, select Tools menu on the contacts window itself and choose the option Export.
3. Name the file and the select destination to save the file. You can also select the type of file as .csv, .vcf, .ldif, and .txt and click Save.
4. Now, open the contacts of your Gmail account from http://contacts.google.com
5. Click on More and select the option Import.
6. Now, select a CSV file you have exported from Thunderbird from the location you have saved.
7. Click on Import and it will import all the contacts from the .csv file.
Thanks & Regards,
Mariya