How to open a file in new tab by default in NERDTree?

Try adding

let NERDTreeMapOpenInTab='\r'

or

let NERDTreeMapOpenInTab='<ENTER>'

to your .vimrc.

s will open the file currently under the cursor in a new vertically split window. Use t to open in a new tab.

You may want to add https://github.com/Nopik/vim-nerdtree-direnter plugin as well - it fixes the directory opening problem, so enter on directory node will just expand/collapse, not open new tab.