The default account option in Thunderbird does not change the e-mail address that new messages will automatically be sent from. Instead, it simply moves the selected account to the top of the folders list.
If this is all that you would like to do, then you can do so using the following steps:
1. Open Thunderbird.
2. From the menu, click on Tool, followed by Account Settings.
3. Locate the account that you wish to use as your default account from the folders list on the left-hand side.
4. Highlight the account name.
5. Click on the Account Actions drop-down field (located at the bottom of the window) and select Set as Default.
6. Click OK and then restart Thunderbird.
Note: Thunderbird will automatically send from the the account that you are viewing when you compose a new message. You can change the sending address by clicking on the From drop-down field (located to the right of the To field) and selecting the appropriate account.