--- a/toolkit/components/places/nsPlacesExpiration.js
+++ b/toolkit/components/places/nsPlacesExpiration.js
@@ -973,17 +973,17 @@ nsPlacesExpiration.prototype = {
       this._timer.cancel();
     if (this._shuttingDown)
       return undefined;
     let interval = this.status != STATUS.DIRTY ?
       this._interval * EXPIRE_AGGRESSIVITY_MULTIPLIER : this._interval;
 
     let timer = Cc["@mozilla.org/timer;1"].createInstance(Ci.nsITimer);
     timer.initWithCallback(this, interval * 1000,
-                           Ci.nsITimer.TYPE_REPEATING_SLACK);
+                           Ci.nsITimer.TYPE_REPEATING_SLACK_LOW_PRIORITY);
     if (this._testingMode) {
       Services.obs.notifyObservers(null, TOPIC_TEST_INTERVAL_CHANGED,
                                    interval);
     }
     return this._timer = timer;
   },
 
   // nsISupports