Issue 5769 - Worked around bad Firefox 55 storage performance

lib/prefs.js

Index: lib/prefs.js
===================================================================
--- a/lib/prefs.js
+++ b/lib/prefs.js
@@ -233,16 +233,53 @@ function keyToPref(key)
   return key.substr(keyPrefix.length);
 }
 
 function prefToKey(pref)
 {
   return keyPrefix + pref;
 }
 
+function savePref(pref)
+{
+  ext.storage.set(prefToKey(pref), overrides[pref]);
+}
+
+let customSave = new Map();
+if (require("info").platform == "gecko")
+{
+  // Saving one storage value causes all others to be saved as well on Gecko.
+  // Make sure that updating ad counter doesn't cause the filters data to be
+  // saved frequently as a side-effect.
+  const MIN_UPDATE_INTERVAL = 60 * 1000;
+  let lastUpdate = -MIN_UPDATE_INTERVAL;
+  let updateScheduled = false;
+  customSave.set("blocked_total", pref =>
+  {
+    if (updateScheduled)
+      return;
+
+    let callback = () =>
+    {
+      lastUpdate = performance.now();
+      updateScheduled = false;
+      savePref(pref);
+    };
+
+    let timeElapsed = performance.now() - lastUpdate;
+    if (timeElapsed < MIN_UPDATE_INTERVAL)
+    {
+      setTimeout(callback, MIN_UPDATE_INTERVAL - timeElapsed);
+      updateScheduled = true;
+    }
+    else
+      callback();
+  });
+}
+
 function addPreference(pref)
 {
   Object.defineProperty(Prefs, pref, {
     get() { return (pref in overrides ? overrides : defaults)[pref]; },
     set(value)
     {
       let defaultValue = defaults[pref];
 
@@ -252,17 +289,17 @@ function addPreference(pref)
       if (value == defaultValue)
       {
         delete overrides[pref];
         ext.storage.remove(prefToKey(pref));
       }
       else
       {
         overrides[pref] = value;
-        ext.storage.set(prefToKey(pref), value);
+        (customSave.get(pref) || savePref)(pref);
       }
     },
     enumerable: true
   });
 }
 
 function init()
 {