fx-prefs.js

var prefs = Services.prefs;
var prefNames = prefs.getChildList("", {});

var prefSizes = {};

for (var i = 0; i < prefNames.length; i++) {
  var prefName = prefNames[i];
  if (prefs.getPrefType(prefName) !== prefs.PREF_STRING) continue;

  prefSizes[prefName] = new Blob([prefs.getStringPref(prefName)]).size;
}

var report = Object.entries(prefSizes)
  .sort(function(a, b) {
    return b[1] - a[1];
  })
  .slice(0, 10)
  .map(function(item) {
    return item[0] + ": " + item[1] + " bytes";
  })
  .join("\n");

console.log(report);