✅ reEnabled download and share images
✅ Enabled HSTS preload list
⛔️ Disabled Telemetry Coverage (hidden pref, I'm not sure that it is also present in fennec but for safety I added it)
⛔️ Set max popups from a single non-click event from 2 to 0 (default 20)
⛔️ Removed limits of the amount of entries in your DNS cache
⛔️ Disabled navigator.registerProtocolHandler (can no longer be used on insecure sites)
(thanks to https://github.com/v1nc for these suggestions)