--- a/netwerk/base/ProxyAutoConfig.cpp
+++ b/netwerk/base/ProxyAutoConfig.cpp
@@ -611,17 +611,17 @@ class JSRuntimeWrapper
   }
 
   bool IsOK()
   {
     return mOK;
   }
 
 private:
-  static const unsigned sRuntimeHeapSize = 2 << 20;
+  static const unsigned sRuntimeHeapSize = 4 << 20; // 4 MB
 
   JSRuntime *mRuntime;
   JSContext *mContext;
   JS::PersistentRooted<JSObject*> mGlobal;
   bool      mOK;
 
   static const JSClass sGlobalClass;