Comment 7 for bug 925603

Alistair,

It seems there are two places where the "default browser" is set. It appears also that chromium automatically sets itself as /etc/alternatives/gnome-www-browser when installed.

So one workaround to make sure that your preference is honored everywhere is, in addition to configuring your browser in System Info -> Default Apps, do this in a terminal:

sudo update-alternatives --config gnome-www-browser

and select the browser you actually want from the shown list.