There is but there might be performance issues with doing so. I'm wondering is this really Firefox's cache taking up all that space? I've always seen Firefox save cache on the computer itself.

Ideas I've got:

• Lower the maximum limit of the cache in the Options > Advanced > Network tab. Checkmark "Override automatic cache management" & set a limit there. Ex. Limit cache to 50 MB of space
• Set Firefox to clear the cache at every shutdown using the "Clear history when Firefox closes" option. Click the Settings button for this option & checkmark "Cache" (Article link)
• Place everyone's Firefox in permanent private browsing mode (no cache, history or passwords are saved in this mode) (Article link)
• Disable Firefox cache with the browser.cache.disk.enable about:config setting and see how that goes (not recommended)

I said not recommended on the last option because you may see weird performance results and other side effects that you won't be able to determine if it was due to disabling that setting.

You can use a mozilla.cfg file in the Firefox program folder to lock prefs or specify new (default) values.

Place a local-settings.js file in the defaults\pref folder where also the channel-prefs.js file is located to specify using mozilla.cfg.

pref("general.config.filename", "mozilla.cfg");
pref("general.config.obscure_value", 0);

These functions can be used in the mozilla.cfg file:

defaultPref();	// set new default value
pref();		// set pref, allow changes in current session
lockPref();	// lock pref, disallow changes

See:

• http://kb.mozillazine.org/Locking_preferences

• http://mike.kaply.com/2012/03/16/customizing-firefox-autoconfig-files/
• http://mike.kaply.com/2014/01/08/can-firefox-do-this/

Adding the following lines to /etc/xul-ext/ubufox.js helped...

pref("browser.cache.disk.enable", false); pref("browser.cache.disk.smart_size.enabled", false); pref("browser.cache.disk.capacity", 0);