Use Command Key to Open Folder in New Window or Tab in Mac OS X
You can open a folder into a new window (or tab) by holding down the Command key when you double-click on the folder to open it within the Finder of Mac OS X.
A new window will open if new windows are set to open in folders rather than tabs, where as a Mac Finder configured to open new folders into new tabs will open a tab instead with the Command modifier. Make sense? If not, just give it a try in the Finder by going to a location with folders, hold down the Command key as you open them, and you’ll quickly understand how it works in just a moment or two.
This is a really useful tip if you’re trying to copy files from inside one folder to another, just open the directories side by side and copy away! So, remember the Command+Double Click trick, it’s great for the Mac Finder.