Follow there steps for mail forwarding in thunderbird
1. Open thunderbird, click on OPTIONS bUTTON.
2. On the message filters windows, click on NEW.
3. Follow the steps in the windows, Like give the name of the filter, then select the condition when the forwarding should be done.
4. On the same windows under "perform these actions, choose the nested action to be performed
5. Once you finish the filter added to the list.