Answering myself:
I have got it fully working now:
Creating a custom button with code:
/*Code*/
// open page in a new tab in the foreground
openContentTab("chrome://chatzilla/content/chatzilla.xul");
opens Chatzilla in a Thunderbird Tab - interestingly the warning message
about the unknown application is also gone...
I found a hint somewhere on the net that the above would work for
Firefox, so I tried it in Thunderbird...
Herwig
Am 07.03.12 17.30, schrieb Wayne Mery: