Mac:Firefox Local Cache

John beat me at rock-paper-scissors so now I have to write this.

We want Firefox to write its caches to the Public folder on the root of the local hard drive. If there isn't already a Public folder, create one.
Open the Applications folder. Right-click (or Control-click) on the Firefox icon and choose Show Package Contents.
Open the Contents folder.
Open the MacOS folder.
Open the defaults folder.
Open the profile folder.
Right-click (or Control-click) on the prefs.js file and choose Open With -> TextEdit. You may have to choose Other... and browse to TextEdit in the Applications folder.

Add the following line to the prefs.js file

user_pref("browser.cache.disk.parent_directory", "/Public/Firefox Cache Local");

Now, delete the user's Firefox profile. It is located in /Users/username/Library/Application Support/Firefox/. Delete the Profiles folder and profiles.ini.
Next time they launch Firefox, it will create a new profile for them with the new cache location setting. You can test this by deleting the local admin's Firefox profile and relaunching Firefox, then checking for the Firefox Cache Local folder & files.