I've been trying to download & install Thunderbird Portable onto a USB drive. But ".exe" doesn't seem to arrive with the download. On one attempt the program itself opened and we went through the account set up; it worked on the original computer but not when the USB was plugged into another computer - "exe. not found" was the reason. Second attempt: it wouldn't start up at all because no .exe found.
The download & install processes were a bit different from the simple instructions on the Thunderbird Portable page, by the way - nothing to double click.