This was the real culprit in terms of RAM bloatage.

pde · pde · commit faad987ad86c · 2012-01-10T18:08:50.000-08:00
Fixes: https://trac.torproject.org/projects/tor/ticket/4804 This core preferences management and logging code needs a serious overhall now that I understand extension JS namespaces a bit better...
diff --git a/src/chrome/content/code/HTTPSRules.js b/src/chrome/content/code/HTTPSRules.js
@@ -36,7 +36,7 @@ function RuleSet(name, match_rule, default_off) {
   this.rules = [];
   this.exclusions = [];
   this.cookierules = [];
-  this.prefs = HTTPSEverywhere.instance.get_prefs();
+  this.prefs = HTTPSEverywhere.instance.prefs;
   try {
     // if this pref exists, use it
     this.active = this.prefs.getBoolPref(name);
diff --git a/src/components/https-everywhere.js b/src/components/https-everywhere.js
@@ -161,6 +161,7 @@ function HTTPSEverywhere() {
   
   var prefs = CC["@mozilla.org/preferences-service;1"]
         .getService(Components.interfaces.nsIPrefService);
+  this.prefs = prefs;
   var branch = prefs.getBranch("extensions.https_everywhere.");
 
   
@@ -228,6 +229,7 @@ const shouldLoadTargets = {
 // HTTP redirects) correctly.
 
 HTTPSEverywhere.prototype = {
+  prefs: null,
   // properties required for XPCOM registration:
   classDescription: SERVICE_NAME,
   classID:          SERVICE_ID,
