Not necessarily the safest thing to do, I agree, but this is how I set up Firefox on my desktop PCs to avoid fragmentation and keep temporary files separate from my OS, Documents, data, and applications.
- Type about:config into the Firefox navigation bar and hit [Enter]
- Right click anywhere and select NEW, then STRING.
- Enter browser.cache.disk.parent_directory in the first popup box
- And then the location in the second popup box, for example: k:\tempintfiles
Information taken from http://www.infohole.com/blog/computing/firefox-cache-location/