Custom provides the same protection as Strict, what Mozilla calls Enhanced Tracking Protection, unless you de-select individual options. In earlier FF versions, ETP was only enabled in Strict/Custom, but was later enforced in all modes. The documentation doesn't describe how Strict or Custom differ from Standard, other than "stronger protection but may cause some sites or content to break".
Enhanced Tracking Protection in Firefox automatically protects your privacy while you browse.
This explains in more detail:
Take back your privacy Firefox is rolling out Total Cookie Protection by default to all Firefox users worldwide, making Firefox the most private and secure