Running xfce4 - and have no gnome control panel - but I needed to change the default browser.
gconftool -g /desktop/gnome/url-handlers/http
gconftool -g /desktop/gnome/url-handlers/https
will tell you what is set - in my case "epiphany %s"
I tend to use opera - so
gconftool -s /desktop/gnome/url-handlers/http -t string 'opera -newpage %s'
gconftool -s /desktop/gnome/url-handlers/https -t string 'opera -newpage %s'
did the trick