diff --git a/mobile/android/base/GeckoAppShell.java b/mobile/android/base/GeckoAppShell.java
index f826853049bb..f222c5a04b0c 100644
--- a/mobile/android/base/GeckoAppShell.java
+++ b/mobile/android/base/GeckoAppShell.java
@@ -323,6 +323,16 @@ public class GeckoAppShell
         if (type != null)
             combinedArgs += " " + type;
 
+        // In un-official builds, we want to load Javascript resources fresh
+        // with each build.  In official builds, the startup cache is purged by
+        // the buildid mechanism, but most un-official builds don't bump the
+        // buildid, so we purge here instead.
+        if (!AppConstants.MOZILLA_OFFICIAL) {
+            Log.w(LOGTAG, "STARTUP PERFORMANCE WARNING: un-official build: purging the " +
+                          "startup (JavaScript) caches.");
+            combinedArgs += " -purgecaches";
+        }
+
         DisplayMetrics metrics = getContext().getResources().getDisplayMetrics();
         combinedArgs += " -width " + metrics.widthPixels + " -height " + metrics.heightPixels;