Well, then the code which chooses a backend is broken because I have xdg-open and, according to WinPdb, it's using the Mozilla backend.

This was my local workaround:

    if os.name == 'posix' and not platform.mac_ver()[0]:
        with open(os.devnull, 'wb') as nul:
            subprocess.Popen(['xdg-open', request_url], stdout=nul, stderr=nul)
    else:
        webbrowser.open_new_tab(request_url)

(This retrofit branch hasn't yet reached the point where it'll be tested on Windows. I may still add another branch which calls `start` directly if it proves to have the same priority bug on Windows.)