By default, Thunderbird displays the tab bar regardless of how many tabs are actually present. By hiding the tab bar when there’s only a single tab, you get a little more real estate displayed on your screen.
1. Go to Preferences.
2. Select the Advanced tab.
3. Click the Config Editor button.
4. Click through the warning.
5. Scroll down to mail.tabs.autoHide and double-click it to toggle the value to true.
6. Close the about:config window.
Your Thunderbird will go from looking like this:
to this:
The tab bar will become visible once there are more than one tab present.
