Use xdg-open or the webbrowser module rather than firefox as default

jozo gave a hint that there are standard tools available to open a webbrrowser:

xdg-open:
* http://portland.freedesktop.org/xdg-utils-1.0/xdg-open.html
* https://wiki.archlinux.org/index.php/Xdg-open

The [webbrowser module](http://docs.python.org/2/library/webbrowser.html)