Start Thunderbird for U3 through an external application

Hi
I use Adobes Lightroom to keep track of my pictures.
In Lightroom there is a solution for sending E-mails with pictures.
Lightroom therefore wants me to point out the exe-file for my standard E-mail application.
Since I uses Thunderbird for U3 on my USB-stick as my email application I tried to point that out. But with no luck
With the exe:s I tried I either I get an error message saying there is something wrong with my Thunderbird-exe or else a new session of Thunderbird starts, thinking it is the first time I start Thunderbird.
I would like to point to the exe that starts when I start it from the launchpad, but I don´t know which exe that is.
Could someone help me find the right one on the U3 stick.

/M