Don't use a static nsCString (causing leaks in the plugin process). Also, use NS_LogInit/NS_LogTerm for the child process so that leaks are reported at the right time.

dom/plugins/PluginModuleChild.cpp

@@ -61,8 +61,6 @@ using namespace mozilla::plugins;
 
 namespace {
 PluginModuleChild* gInstance = nsnull;
-// NPN_UserAgent returns a |const char*| that must outlive its stack frame
-nsCString gUserAgent;
 }
 
 
@@ -205,6 +203,15 @@ PluginModuleChild::CleanUp()
 {
 }
 
+const char*
+PluginModuleChild::GetUserAgent()
+{
+    if (!CallNPN_UserAgent(&mUserAgent))
+        return NULL;
+
+    return NullableStringGet(mUserAgent);
+}
+
 bool
 PluginModuleChild::RegisterNPObject(NPObject* aObject,
                                     PluginScriptableObjectChild* aActor)
@@ -765,8 +772,7 @@ _useragent(NPP aNPP)
     _MOZ_LOG(__FUNCTION__);
     AssertPluginThread();
 
-    PluginModuleChild::current()->CallNPN_UserAgent(&gUserAgent);
-    return NullableStringGet(gUserAgent);
+    return PluginModuleChild::current()->GetUserAgent();
 }
 
 void* NP_CALLBACK

dom/plugins/PluginModuleChild.h

@@ -137,6 +137,8 @@ public:
 
     void CleanUp();
 
+    const char* GetUserAgent();
+
     static const NPNetscapeFuncs sBrowserFuncs;
 
     static PluginModuleChild* current();
@@ -157,6 +159,7 @@ private:
 
     std::string mPluginFilename;
     PRLibrary* mLibrary;
+    nsCString mUserAgent;
 
     // we get this from the plugin
 #ifdef OS_POSIX

toolkit/xre/nsEmbedFunctions.cpp

@@ -287,9 +287,13 @@ XRE_InitChildProcess(int aArgc,
 
   base::AtExitManager exitManager;
 
+  NS_LogInit();
+
   int rv = XRE_InitCommandLine(aArgc, aArgv);
-  if (NS_FAILED(rv))
-      return NS_ERROR_FAILURE;
+  if (NS_FAILED(rv)) {
+    NS_LogTerm();
+    return NS_ERROR_FAILURE;
+  }
 
   MessageLoopForIO mainMessageLoop;
 
@@ -331,6 +335,7 @@ XRE_InitChildProcess(int aArgc,
     sIOMessageLoop = nsnull;
   }
 
+  NS_LogTerm();
   return NS_OK;
 }