I had the same problem and found The solution here:
http://www.infohole.com/blog/computing/ ... -location/Type about:config into the firefox navigation bar and hit enter
Find the browser.cache.disk.parent_directory, if it doesn't exist right click anywhere and select NEW, then STRING. Enter browser.cache.disk.parent_directory in the first popup box and then the location in the 2nd popup box
e.g. C:\Documents and Settings\<user>\AppData\Local\Temp\
If the entry already existed simply right click on it and select MODIFY and enter your new directory path.
Now your firefox cache location has been changed!
Ahh¡¡ I also need to activate start a HtppFox