linux doesn't really use "portable" apps.
there's ways to run tor browser in its own environment from a usb stick; there was a program that would do that automatically, but i can't find it anymore :-(
for me, the bigger question is: Why? since you have ubuntu installed anyway, and since tor browser is designed to leave no traces on your computer?
but, i found this: http://portablelinuxapps.org/
- seems legit. wasn't the one i remembered, though.