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

[user1]

> There's a gui for that:
> http://packages.ubuntu.com/lucid/galternatives $ sudo apt-get install
> galternatives
> then: Applications|System Tools|Alternatives Configurator

I tried that, added /usr/bin/icecat to the x-www-browser line and gave it 
priority 110 (firefox has priority 40) but firefox still opens as default 
browser when clicking links in thunderbird.