webbrowser.open(), webbrowser.open_new(), and webbrowser.open_new_tab() all do the exact same thing, regardless of the flags that I set. In Firefox, open('www.google.com', new=0), open_new('www.google.com'), and open_new_tab('www.google.com') all open either three new www.google.com tabs (if "Open new windows in a new tab instead" is selected in FF options) or three new www.google.com windows (if "Open new windows in a new tab instead" is not selected in FF options). In Internet Explorer, three new www.google.com tabs are created.

The issue exhibits itself the same way whether or not I have the browser open before running the script.

Environment was a Windows Vista 32-bit machine, running Python 3.1.2.

Example script reads:
import webbrowser
import time
ff = webbrowser.get('firefox')
ff.open('www.google.com', new=0)
time.sleep(3)
ff.open_new('www.google.com')
time.sleep(3)
ff.open_new_tab('www.google.com')