I have opensuse 42.1 and I selected to install the xfce4 desktop. From the settings menu I changed the default browser from Firefox to seamonkey and it had no effect. I had a memory of something like this happening before, so I installed gnome-control-center and changed the default browser in that, and voila! it worked! So I am curious to know why this is? Maybe xfce4 should have a dependency for gnome-control-center instead of whatever it installs by default.