Many of these sites suggest making edits via “about:config”. The problem is it’s difficult to keep track of which settings you’ve altered. I highly recommend you use a user.js [1] file. The best part is you can make notes so later you knows why a setting was enabled or disabled.
The only downside is if you decide to undo a setting in user.js you’ll also have to make the same change in about:config.
Firefox's about:support page lists the about:config prefs that have non-default values ("Important Modified Preferences"). That includes prefs you've changed and internal prefs changed by Firefox code.
Unfortunately the info provided by about:support doesn’t separate changes made via the gui and changes made via about:config. It also includes settings that may have been altered by Add-ons.
Another benefit to using a user.js file is that your changes are persistent. If Mozilla changes a setting via an upgrade, like they did with experiments and plan to do with DoH, your changes aren’t overridden.
They are saved under the user profile directory in the `prefs.js` file. src[0] I have it symlinked to `~/.config/firefox/pref.js` (which also contains `chrome/userChrome.css`, also symlinked to from the firefox profile) in this way they get checked into git with the rest of my dotfiles.
As long as `chrome://global/content/config.xul` still exists, you can use it. I certainly do, since the new interface is horrible and Mozilla constantly dismisses the obvious usability issues.
The only downside is if you decide to undo a setting in user.js you’ll also have to make the same change in about:config.
[1] http://kb.mozillazine.org/User.js_file