How can I set the Cache to my FirefoxPortable folder?

Installed the latest Firefox Portable today. Loving how well it does everything. My concern: I'm using the portable to avoid Firefox writing data to my C:\ drive. I have the program installed on D:\ in D:\Utilities II\FirefoxPortable, and I'd like to direct it to put the daily cache files in a directory in that folder, say, \Other\cache. I can't find help online that dates within this century (!), and I'm hoping someone here can help me out.