Bug with Thunderbird 3ß4 or Windows 7 -- relatively minor

I am not sure where this bug should go, because I have changed too many things at once. My office computer now uses Windows 7 64-bits, and I read e-mail on Thunderbird 3 beta 4 Portable installed on PortableApps 2 beta 3.

I know I only need to click once on programs listed in the PortableApps window; but sometimes I doubleclick by error. When I doubleclick on Thunderbird Portable, I get the following error message (translated from French):
"You are trying to open Thunderbird from a read-only location... This does not work."
Then, it closes.

Now I realise that the message actually comes from the second instance of Thunderbird I was trying to open (the second click), because if I wait long enough, the first instance (?) of Thunderbird Portable starts as it should. And the error only happens when I double-click. In other words, trying to re-open Thunderbird Portable when it is already running switches me to the Thunderbird Portable window, as it should.

So my questions/comments are:
– Is the error message generated by Thunderbird Portable, PortableApps, or Windows 7?
– Is it possible to change the error message to something more accurate like "Another instance of Thunderbird Portable is already running."