Portable Thunderbird 2.0.0.21 doesn't find K-Meleon 1.5.3

Recently I switched from Firefox to K-Meleon as my default browser. And although K-Meleon is reported to Windows XP as my default browser, Portable Thunderbird doesn't open it when clicking on a hyperlink.

How can I let Portable Thunderbird know that it has to open K-Meleon? Both programs are not used 'portable'; I installed them on my hard disk into my desktop PC.

Thank you very much for your help, regards, Daniel