why not configure for to see if the entry user_pref("browser.cache.disk.parent_directory", "D:\\TEMP\\Firefox\\Cache"); exists in the file prefs.js ?, and if exists use it
advance user prefer put cache folder in other HD different from OS for many reseason: more clean OS partition, less defrag HD, more speed to acces data (access simultaneous to HD OS and cache HD)....
For example I use a partintion in other HD for all cache and temp folder: pagefile.sys, windows temps, IE temp, firefox cache, and other apps.
Sorry for my bad English.