[SOLVED] Temporary file directory not portable

Using Mozilla Thunderbird Portable version 2.0.0.23 (20090812)
on a the user account of a locked-down company computer.

When I attempt to send a message or save it as draft,
I get the following error in a modal dialog box :

"Unable to save your message as draft
Error writing temporary file"

Some other threads as suggested that the error might be caused
by the Antivirus (Symantec in my case) blocking the write attempt,
but I suspect it might be a case of insufficient permissions.

Support threads elsewhere have hinted that on Windows,
Thunderbird uses the system's temporary directory.

For my user, the variables that set the temporary directories are :

TEMP : %USERPROFILE%\Local Settings\Temp
TMP : %USERPROFILE%\Local Settings\Temp

Globally on this system, they are :

TEMP : C:\WINDOWS\TEMP
TMP : C:\WINDOWS\TEMP

I suspect that Thunderbird uses the globally set
temporary directory instead of the one defined for my user.
And moreover, to remain portable, shouldn't it stick to
somewhere inside the local PortableApps hierarchy ?

I have found no way to set which temporary directory
Thunderbird uses, not even in the advanced configuration editor.
No hint about that on the Web either.

Does anyone here have a clue about this problem ?