Scratch that. I was looking for the wrong about:config value. While you can't have private browsing mode be the default, it is in fact possible to enable track

Scratch that. I was looking for the wrong about:config value. While you can't have private browsing mode be the default, it is in fact possible to enable tracking protection outside private browsing mode, so you can get these same benefits (privacy.trackingprotection.enabled in about:config). Wish this would be exposed in the UI settings.