By default this shouldn't do anything different from before. However, 
with this patch, functions for 'browse-url-browser-function' can look at 
the argument (as modified by 'browse-url-new-window-flag') and do 
something special. For example, with this patch, I have a custom browser 
function that works like this:

   M-x browse-url      Open in Firefox
   C-u M-x browse-url  Open in EWW
   C-- M-x browse-url  Open in Firefox Private Browsing