How to stop Firefox from loading local memory copy of page?

I have clients who say emails they deleted keep reappearing after they load Firefox. This is clearly the local-disk preload feature (I don’t know the proper name offhand) that is supposed to make Firefox appear faster. I have encountered this issue myself numerous times and I don’t know how to disable it.

What is the about:config option to disable loading saved memory images of pages?

This is explicitly not a web developer “disable all cache” question.