qutIM follows freedesktop.org specifications, so tries to use "xdg-open" app.
You should tune it instead of reinventing the wheel at all other applications.
If you doesn't use Gnome or KDE and doesn't have "xdg-open" firefox will always be default one untill you fully remove it from system
Может потому что огнелис выбран дефолтным браузером в вашей системе?