I’m titling this the way it seemed to me back when I was wondering what was going on. In fact the issue is that system problems are reported with root privileges, and Opera had hijacked the system-level default browser position (my normal account still used my choice). You can see which browser is the superuser default by issuing
$ sudo xdg-open 'http://www.google.com'
You can configure the default with
$ sudo update-alternatives --config gnome-www-browser
$ sudo update-alternatives --config x-www-browser