Why can't the Cache Size Variables be changed via the FF Options? This Feature was available in Older Versions!
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Current Firefox releases use automatic cache management (browser.cache.disk.smart_size.enabled). You will have to modify related browser.cache.disk prefs directly on the about:config page if you do not want to use the defaults.
You can open the about:config page via the location/address bar. You can click the button to "Accept the Risk and Continue".