How to enable cache and set it to be at the FXPortable directory

I'm using Firefox Portable in a TrueCrypt-encrypted file container. This functions exactly like a flash disk (once a password is supplied and the file container is opened, it is mounted by TrueCrypt as a drive at My Computer.) Since it is actually in a hard disk, and not in a flash drive, reads and writes are not an issue. Because of this, I would like to enable Firefox Portable's cache.

How do I set Firefox Portable to have its cache enabled and have it in the Firefox Portable directory?