Did you try adding the option yourself?

After you open the config editor (Edit --> Preferences --> Advanced--> Make sure you are in the General tab --> Config Editor), right click inside the new window.

In the context menu, choose New to open a submenu and click on String.

You will be asked to enter the preference name, put in browser.cache.disk.parent_directory and click Ok. Next you will be asked to enter the value, which is the path you wish to use to store the cache.

It will create a directory called Cache inside the path you provide in the option, where it will store the cache files.

P.S. It is browser.cache.disk.parent_directory and not browser.cache.parent_directory