1. Click on the Tools menu and
2. Select Account Settings.
3. Click on the Server Settings tab under the first email account you want to change.
4. The mail checking frequency is set with Check for new messages every __ minutes. Check the box next to this item and fill in an appropriate number of minutes.
5. Click OK to save changes.
From now on, Thunderbird will check for new mail in that specific account as frequently as you have selected.
Repeat the steps above for other email accounts you have set up.
