Digital Media Minute did a post a while ago about the location of the Firefox cache.
To change Firefox cache location, add the “browser.cache.disk.parent_directory” preference (by editing your configuration) and enter a folder path. When you specify this preference, a new directory, “Cache” will be created at the path you have set.
These other two preferences must set as follows (edit configuration):
Set “browser.cache.disk.enable” to ‘true’ to store cache on the hard drive. (The default is true.)
Set “browser.cache.disk.capacity” to a positive integer equal to the max hard drive space in KB to use for cache. (Firefox default is 50000 KB)
Also, for Windows, the path separator must be 2 backslashes (for example “C:\\Path\\To\\Parent”), if the “browser.cache.disk.parent_directory” value is set in prefs.js or user.js file rather than about:config.
Changing the cache location in Firefox might be something you want to do if you would rather route cache data to an encrypted hard drive rather than leaving it on the drive to which it’s currently being stored even after you clear your cache.