I think you’re mixing up Waterfox with Librewolf. Librewolf by default deletes all your data after closing it, Waterfox doesn’t.
It’s actually for this reason that I use both for different purposes. I sign into accounts with Waterfox (using account containers and browser profiles so each account’s data is self-contained and not shared between each other) and use a sandboxed installation of Librewolf with a few extensions like NoScript installed and letterboxing enabled for anything I don’t sign into (i.e. news articles, web searches, etc.) so that the 50-odd domains running scripts on my local news station’s website get way less of a fingerprint off me to use to target ads at me or sell my data to data brokers or the gestapo.
Lucky. The only solar panels in my local retail parking lots are the ones powering the ALPRs they installed there.