How to set icecat as default browser when clicking links in thunderbird

[user1]

I think I have tried them all, even in gnome.
 
> $ sudo update-alternatives --config x-www-browser

sudo update-alternatives --config x-www-browser
There are 3 choices for the alternative x-www-browser (providing /usr/bin/
x-www-browser).

  Selection    Path                       Priority   Status
------------------------------------------------------------
* 0            /usr/bin/konqueror          100       auto mode
  1            /usr/bin/chromium-browser   40        manual mode
  2            /usr/bin/firefox            40        manual mode
  3            /usr/bin/konqueror          100       manual mode

Press enter to keep the current choice[*], or type selection number: 

I see no icecat?