url click should open link in browser, in Thunderbird

Hi all:

One of the persons i helped install and setup Thunderbird for
email communication is facing the following issue:

When the individual clicks on a URL link in HTML mail, browser
does not startup.

Installed software are
 . Firefox 3.0.x
 . Thunderbird 2.0.0.21

How can do i help configure 'firefox' startup on click in
Thunderbird ?

Thanks in advance.


thanks
Saifi.