OS X:
- Open the Safari plist file (~/Library » Preferences » com.apple.Safari.plist) in the Property List Editor.
- Click the triangle to open the Root list, then click once on Root to highlight it.
- Click the Add Child button.
- Change the name (New item) to AlwaysShowTabBar, set the Class to Boolean, and set Value to Yes.
- Save changes and quit Property List Editor.
Windows:
For Windows, manually add the below key into the Preferences.plist:
<key>AlwaysShowTabBar</key>
<true/>
This hint tells you where to find Safari's plist in Windows.

