About:config saving in account settings

Put the changes in a user.js file in your Firefox profile directory (the same one that probably already has prefs.js and logins.js).

Use this format (same as about:config):

user_pref("dom.security.https_only_mode", true);
user_pref("ui.systemUsesDarkTheme", 1);

Then sync or backup the whole directory, excluding the "cache" subdirectories to save space.

(Pet peeve: Firefox, please use $HOME/.cache/ like every other app!)

You might try losing everything but the .json and .sqlite files. Have not checked, but that is probably enough. Only missing paths are regenerated when you launch Firefox.