If you want Firefox not to remember your Internet data, go to Tools-Privacy & Security- scroll down to History- in the drop down menu to the right of "Firefox will", choose "Never Remember History". At least that is the case for the Waterfox version I use. In the past I played around with memory cache settings in Firefox "about:config" settings and found it didn't always work, I'm not sure why. I finally grew tired of Firefox data collection and went to Waterfox.