Aha! The setting is correctly controlled by gnome-default-applications-properties. Running this (or launching 'Preferred Applications' from a menu/Unity) contains a "Web Browser" setting that was set to Firefox. Changing this to Chromium did the right thing.
I'm not at all sure what gnome-default-applications-properties is setting that the browsers aren't, but that seems to be the solution.
Aha! The setting is correctly controlled by gnome-default- applications- properties. Running this (or launching 'Preferred Applications' from a menu/Unity) contains a "Web Browser" setting that was set to Firefox. Changing this to Chromium did the right thing.
I'm not at all sure what gnome-default- applications- properties is setting that the browsers aren't, but that seems to be the solution.