[link|http://kb.mozillazine.org/Browser.cache.disk.parent_directory|Mozillazine]
The Cache location can be changed by adding the "browser.cache.disk.parent_directory" preference and entering a new path (Note: In the past, this was specified in the "browser.cache.disk.directory" preference and before that, "browser.newcache.directory" and "browser.cache.directory" behaved similarly.)

I've never had a reason to try this, but it should work.

Jay