Configuring preferred web browser with Icedove

[AG <computing.account@googlemail.com>]

Hello List

How does one configure Icedove (Thunderbird) to use the system's default 
browser.  My system's default browser is Opera, but Icedove will always 
call up Konqueror and I cannot see any obvious way to change that ... 
even "advanced config" doesn't seem to show any options.

I'm using Squeeze up-to-date.

Thanks


AG