I don't use or recommend exactly what they do but they have a long list of settings you might want to change that is better documented than any other source I've seen and if you click the wiki link there is detailed info about how user.js works.
You can start with mine, I've mostly stripped all the telemetry, added DoQ DNS (please, don't use my NextDNS id :D otherwise it's going to eat the free 300k limit), lots extra DNS/HTTP/rendering performance tweaks, some security fixes (e.g. deprecated ciphers) without performance penalties and personal Firefox's quality of life changes (smaller delays, ability to save everything, tracker stripping, etc). Some configs like the already mentioned Arkenfox's take security to a next level with first party cookies only, sandboxing and etc, it might be too overwhelming and not actually that necessary.
