browser.cache.disk.parent_directory in firefox

[Rich Winkel]

I'd like to move firefox's cache dir into tmp, under a user-specific
parent directory.  How can I specify the userid in the argument to
browser.cache.disk.parent_directory in the system-wide firefox.js settings
file?
Something like
user_pref("browser.cache.disk.parent_directory", "/tmp/ffCache/$USER");

??

Thanks,
Rich