It's too long to paste as a comment here, but I started by finding every entry with a URL and deleting the URL (leaving it blank) for basically any domain that wasn't mozilla.tld (careful here though) and then I used the recommended changes from arkenfox, ghacks, the TOR browser and https://support.mozilla.org/en-US/kb/how-stop-firefox-making... and then tweak from there.
I disable things like service workers, normandy, shield, pocket, the new tab page, WebRTC, searching from the address bar, prefetch, WebGL, push notifications, WebAssembly, and lots of dom. options
You can backup your user.js but updates tend to add/rename/remove preferences so that gets ugly fast. I've also seen firefox reset prefs I've set but didn't lock. I've been meaning to automate monitoring the file for changes and notifying me for lines that were added or modified, but I haven't gotten around to it yet.
