Do you mean this?
Specify where to store the cache
To specify in which folder the cache is stored, add the following code to your user.js file:
// Path to Cache folder:
user_pref("browser.cache.disk.parent_directory","C:\\Path To Cache");
Remember to use two backslashes for the path separators if you're using Windows, e.g. "C:\\Path\\Path To Cache" instead of "C:\Path\Path To Cache".
From
here.