You sir, fail at Google. This setting has been available from the beginning!

Type "about:config" into the address bar.

In the filter, type "cache" to see the settings related to caching.

There's a setting there to completely turn off the cache if you like: "browser.cache.disk.enable"

To choose the folder, create a setting "browser.cache.disk.parent_directory" and set it to whatever you want.