In Thunderbird, open your Address Book by clicking “Tools – Address Book”.
Select the addresses you want to migrate, or Ctrl-A to select them all. Then save them to a CSV (Comma Separated) file by clicking “Tools – Export”. Save the file somewhere that you find it again.
In Google Mail, click on “Contacts” at the top-left of the page, or at the bottom of the list of labels/folders on the left.) Then click “Import”. Select the CSV file that you created in the step above, and let Google import it into your Contacts list.
Note that in Google Mail, your address book is loaded when Mail starts up. If you import new addresses, you'll need to restart Google Mail for it to “see” them. You can just close your browser and login again.