Users can mark privacy-sensitive sites (banking, email) as "Exclude from memory cache" — those responses go only to disk or not cached at all. This uses the underlying capacity preference but adds site-level exceptions.
Here is a built around exposing and enhancing this preference for a privacy-focused or developer-oriented browser: Browser.cache.memory.capacity
In all other cases, trust Firefox to handle its own memory—your browsing will be faster and more stable for it. Users can mark privacy-sensitive sites (banking, email) as