I just wrote up a short batch script that uses 7zip's encryption to pack up portablefirefox's data folder. Since running firefox from the USB gives such a perfomance hit, my script basically copies the portablefirefox folder to c:\temp & extracts the data folder. When I'm done browsing, I tell it to pack up the data folder back to my thumb drive & delete everything else. It can use a keyfile, or a manual password as I want it to.
I imagine such a thing would be easy to officially implement. It's only a 10 line script which anybody with a bit of programming experience could do. I can post it here if anyone wants it. Any chance something like it could be added to the official version?