--- a/mobile/android/app/mobile.js
+++ b/mobile/android/app/mobile.js
@@ -734,20 +734,16 @@ pref("media.plugins.enabled", true);
 //  0 = Let Stagefright choose hardware or software decoding (default)
 //  8 = Force software decoding
 // 16 = Force hardware decoding
 pref("media.stagefright.omxcodec.flags", 0);
 
 // Coalesce touch events to prevent them from flooding the event queue
 pref("dom.event.touch.coalescing.enabled", false);
 
-// On memory pressure, release dirty but unused pages held by jemalloc
-// back to the system.
-pref("memory.free_dirty_pages", true);
-
 pref("layout.framevisibility.numscrollportwidths", 1);
 pref("layout.framevisibility.numscrollportheights", 1);
 
 pref("layers.enable-tiles", true);
 
 // Enable the dynamic toolbar
 pref("browser.chrome.dynamictoolbar", true);
 

--- a/modules/libpref/init/all.js
+++ b/modules/libpref/init/all.js
@@ -5201,18 +5201,19 @@ pref("memory.low_physical_memory_thresho
 pref("memory.low_memory_notification_interval_ms", 10000);
 #endif
 
 // How long must we wait before declaring that a window is a "ghost" (i.e., a
 // likely leak)?  This should be longer than it usually takes for an eligible
 // window to be collected via the GC/CC.
 pref("memory.ghost_window_timeout_seconds", 60);
 
-// Disable freeing dirty pages when minimizing memory.
-pref("memory.free_dirty_pages", false);
+// On memory pressure, release dirty but unused pages held by jemalloc
+// back to the system.
+pref("memory.free_dirty_pages", true);
 
 // Disable the Linux-specific, system-wide memory reporter.
 #ifdef XP_LINUX
 pref("memory.system_memory_reporter", false);
 #endif
 
 // Don't dump memory reports on OOM, by default.
 pref("memory.dump_reports_on_oom", false);

--- a/xpcom/base/AvailableMemoryTracker.cpp
+++ b/xpcom/base/AvailableMemoryTracker.cpp
@@ -328,34 +328,34 @@ public:
   void Init();
 
 private:
   static bool sFreeDirtyPages;
 };
 
 NS_IMPL_ISUPPORTS(nsMemoryPressureWatcher, nsIObserver)
 
-bool nsMemoryPressureWatcher::sFreeDirtyPages = false;
+bool nsMemoryPressureWatcher::sFreeDirtyPages = true;
 
 /**
  * Initialize and subscribe to the memory-pressure events. We subscribe to the
  * observer service in this method and not in the constructor because we need
  * to hold a strong reference to 'this' before calling the observer service.
  */
 void
 nsMemoryPressureWatcher::Init()
 {
   nsCOMPtr<nsIObserverService> os = services::GetObserverService();
 
   if (os) {
     os->AddObserver(this, "memory-pressure", /* ownsWeak */ false);
   }
 
   Preferences::AddBoolVarCache(&sFreeDirtyPages, "memory.free_dirty_pages",
-                               false);
+                               true);
 }
 
 /**
  * Reacts to all types of memory-pressure events, launches a runnable to
  * free dirty pages held by jemalloc.
  */
 NS_IMETHODIMP
 nsMemoryPressureWatcher::Observe(nsISupports* aSubject, const char* aTopic,