sudo update-alternatives --config gnome-www-browser
There are 4 choices for the alternative gnome-www-browser (providing /usr/bin/gnome-www-browser).

  Selection    Path                       Priority   Status
------------------------------------------------------------
* 0            /usr/bin/opera              120       auto mode
  1            /usr/bin/chromium-browser   40        manual mode
  2            /usr/bin/epiphany-browser   85        manual mode
  3            /usr/bin/firefox            40        manual mode
  4            /usr/bin/opera              120       manual mode

Press <enter> to keep the current choice[*], or type selection number: 3
update-alternatives: using /usr/bin/firefox to provide /usr/bin/gnome-www-browser (gnome-www-browser) in manual mode