browse-url-netscape-new-window-is-tab

browse-url-netscape-new-window-is-tab is a variable defined in `browse-url.el'.
Its value is

nil

Documentation:
Whether to open up new windows in a tab or a new window.
If non-nil, then open the URL in a new tab rather than a new
window if `browse-url-netscape' is asked to open it in a new
window.

You can customize this variable.