[PhantomGremlin]

a thing you're supposed to manually enter

Doesn't everybody know about user.js? Keep that updated and store that in git. If that file is present in the .../Profiles/<whatever>.default then Firefox automatically uses it. And Firefox doesn't rewrite it, as opposed to prefs.js.

It's still a hassle, but it's far easier than manually entering things into about:config. Here are a few of the things currently in my user.js:

   user_pref("accessibility.blockautorefresh", true);
   user_pref("browser.preferences.inContent", false);
   user_pref("network.dns.disablePrefetch", true);
   user_pref("network.prefetch-next", false);
   user_pref("pdfjs.disabled", true);
   user_pref("plugins.hide_infobar_for_blocked_plugin", true);
   user_pref("plugins.notifyMissingFlash", false);
   user_pref("social.enabled", false);
   user_pref("social.remote-install.enabled", false);

I've got more in there but I'm too lazy to look all of them up to make sure they still apply to the current version of Firefox. The ones I pasted seemed relevant based on their name.

[technomancy]

Sure, but you can't check the profile directory into git, and you can't know the path of the profile directory ahead of time to make a symlink either. The way they randomize the path makes it seem like they're going out of their way to make it automation-resistant.

[Animats]

When you have to do something like that to disable unwanted intrusive and commercial features in Firefox, you know that the Mozilla Foundation is not your friend.