How to change firefox cache directory

From here

1. In Windows XP, go to C:\Documents and Settings\XXX\Application Data\Mozilla\Firefox\Profiles\XXXXX.default

2. Create a file called User.js (same directory as Prefs.js)

3. Paste this into it and save:
// Path to Cache folder:
user_pref("browser.cache.disk.parent_directory","x :\\");

X:\\ is the directory you want to place the cache

Useful when you want to place the cache onto a ramdisk