Default for GNOME sessions (system-wide)
The default GNOME browser can be changed for the whole system by reconfiguring the alternative.
update-alternatives --config gnome-www-browser
Default for foreign programs (system-wide)
Programs which are not designed for the user’s desktop environment do not obey the browser settings of the desktop environment (GNOME or KDE). For example, Icedove ignores desktop environment-specific browser settings. The default browser for generic applications can be changed for the whole system by reconfiguring the x-www-browser alternative.
update-alternatives --config x-www-browser