Thunderbird Writes to TEMP Folder...how to stop?

You must set the TMP environment variable to the desired folder. Put a batch file alongside Thunderbird Portable's executable. Insert the following contents in it:

@echo off
set TMP=%~dp0temp-folder
"%~dp0ThunderbirdPortable.exe"

Now all you have to do is run the batch file instead of ThunderbirdPortable.exe. The modified environment variables are only valid for the current cmd.exe session, meaning this won't change the place where other apps store their temporary files.

(%~dp0 simply gets the full path of the batch file, including drive letter.)

Almost forgot. You also need to enable command extensions.

Add this as the second line of the previous batch script:

setlocal enableextensions

And this as the last line:

endlocal