Download link for 38.1.0 on main page might be wrong?
The download link on https://portableapps.com/apps/internet/thunderbird_portable that points to https://downloads.sourceforge.net/portableapps/ThunderbirdPortable_38.1.... doesn't seem to go anywhere. I initially thought it was due to script blocking addons in my FF install so I tried it in Google Chrome which doesn't run with any addons and it behaved the same.