In MATLAB 6.5, there was a preference setting for tabbed/untabbed behavior in the MATLAB Editor/Debugger. This preference is no longer listed in the "Preferences -> Editor -> Display" options list.
In MATLAB 7.0 and later versions, this information is an "implicit preference". MATLAB remembers the selection you made in the previous session, and continues that behavior. So, to set an "implicit preference" for separate, undocked windows, take the following steps:
1. Open a file in the Editor/Debugger.
2. Undock the file from the Editor/Debugger using either the undock button in the upper right corner or the "Desktop -> Undock <filename>" item.
3. Close the empty Editor/Debugger that is left behind after undocking the file.
4. Subsequently opened files will appear in individual undocked windows rather than being "tabbed" together. This implicit preference will be remembered across sessions and will remain in effect indefinitely unless you explicitly dock a file back into the Editor/Debugger.
To cause MATLAB to open files and automatically dock them in the Editor/Debugger, dock one file by clicking on the Dock button (an arrow pointing down and to the right) in the upper-right corner of the Editor/Debugger.
The Editor/Debugger also remembers other view settings, including the document arrangement selected from the "Window" menu or from the buttons in the upper-right corner of the Editor/Debugger window.