If you installed the latest Thunderbird beta you will notice pressing the enter key on a message will open it in a new tab instead of a new window. Here is how to fix this.
1. In Thunderbird click tools>Options>advanced tab.
2. Click the Config Editor butten.
3. Read the message about how this may void your warrenty and then Click the I’ll be careful button.
4. In the filter edit box type “mail.openMessageBehavior” and the list box will be filtered down to two items. “mail.openMessageBehavior”, and “mail.openMessageBehavior.version”.
5. We will modify “mail.openMessageBehavior” by right clicking on this item and choose Modify from the context menu.
6. Replace the #2 with #1 and press the OK butten.
7. Close the configuration editor.
8. Press the OK butten on the options screen.
9. Exit and restart Thunderbird to make sure the new setting takes effect.
You can now press the enter key on a message and the message will now be opened in a new window.