diff --git a/modules/libpref/src/nsPrefBranch.cpp b/modules/libpref/src/nsPrefBranch.cpp
index 423caff55dd..46bfcb0cd25 100644
--- a/modules/libpref/src/nsPrefBranch.cpp
+++ b/modules/libpref/src/nsPrefBranch.cpp
@@ -44,6 +44,7 @@
 #endif
 
 #include "nsPrefBranch.h"
+#include "nsPrefService.h"
 #include "nsILocalFile.h"
 #include "nsIObserverService.h"
 #include "nsXPCOM.h"
@@ -52,6 +53,7 @@
 #include "nsString.h"
 #include "nsReadableUtils.h"
 #include "nsXPIDLString.h"
+#include "nsThreadUtils.h"
 #include "nsIStringBundle.h"
 #include "prefapi.h"
 #include "prmem.h"
@@ -147,6 +149,10 @@ NS_INTERFACE_MAP_END
 
 NS_IMETHODIMP nsPrefBranch::GetRoot(char **aRoot)
 {
+  if (NS_UNLIKELY(!nsPrefService::CheckAndLogBackgroundThreadUse())) {
+    return NS_ERROR_UNEXPECTED;
+  }
+
   NS_ENSURE_ARG_POINTER(aRoot);
   mPrefRoot.Truncate(mPrefRootLength);
   *aRoot = ToNewCString(mPrefRoot);
@@ -155,6 +161,10 @@ NS_IMETHODIMP nsPrefBranch::GetRoot(char **aRoot)
 
 NS_IMETHODIMP nsPrefBranch::GetPrefType(const char *aPrefName, PRInt32 *_retval)
 {
+  if (NS_UNLIKELY(!nsPrefService::CheckAndLogBackgroundThreadUse())) {
+    return NS_ERROR_UNEXPECTED;
+  }
+
   NS_ENSURE_ARG(aPrefName);
   const char *pref = getPrefName(aPrefName);
   *_retval = PREF_GetPrefType(pref);
@@ -163,6 +173,10 @@ NS_IMETHODIMP nsPrefBranch::GetPrefType(const char *aPrefName, PRInt32 *_retval)
 
 NS_IMETHODIMP nsPrefBranch::GetBoolPref(const char *aPrefName, PRBool *_retval)
 {
+  if (NS_UNLIKELY(!nsPrefService::CheckAndLogBackgroundThreadUse())) {
+    return NS_ERROR_UNEXPECTED;
+  }
+
   NS_ENSURE_ARG(aPrefName);
   const char *pref = getPrefName(aPrefName);
   return PREF_GetBoolPref(pref, _retval, mIsDefault);
@@ -170,6 +184,10 @@ NS_IMETHODIMP nsPrefBranch::GetBoolPref(const char *aPrefName, PRBool *_retval)
 
 NS_IMETHODIMP nsPrefBranch::SetBoolPref(const char *aPrefName, PRInt32 aValue)
 {
+  if (NS_UNLIKELY(!nsPrefService::CheckAndLogBackgroundThreadUse())) {
+    return NS_ERROR_UNEXPECTED;
+  }
+
 #ifdef MOZ_IPC
   if (GetContentChild()) {
     NS_ERROR("cannot set pref from content process");
@@ -184,6 +202,10 @@ NS_IMETHODIMP nsPrefBranch::SetBoolPref(const char *aPrefName, PRInt32 aValue)
 
 NS_IMETHODIMP nsPrefBranch::GetCharPref(const char *aPrefName, char **_retval)
 {
+  if (NS_UNLIKELY(!nsPrefService::CheckAndLogBackgroundThreadUse())) {
+    return NS_ERROR_UNEXPECTED;
+  }
+
   NS_ENSURE_ARG(aPrefName);
   const char *pref = getPrefName(aPrefName);
   return PREF_CopyCharPref(pref, _retval, mIsDefault);
@@ -191,6 +213,10 @@ NS_IMETHODIMP nsPrefBranch::GetCharPref(const char *aPrefName, char **_retval)
 
 NS_IMETHODIMP nsPrefBranch::SetCharPref(const char *aPrefName, const char *aValue)
 {
+  if (NS_UNLIKELY(!nsPrefService::CheckAndLogBackgroundThreadUse())) {
+    return NS_ERROR_UNEXPECTED;
+  }
+
 #ifdef MOZ_IPC
   if (GetContentChild()) {
     NS_ERROR("cannot set pref from content process");
@@ -206,6 +232,10 @@ NS_IMETHODIMP nsPrefBranch::SetCharPref(const char *aPrefName, const char *aValu
 
 NS_IMETHODIMP nsPrefBranch::GetIntPref(const char *aPrefName, PRInt32 *_retval)
 {
+  if (NS_UNLIKELY(!nsPrefService::CheckAndLogBackgroundThreadUse())) {
+    return NS_ERROR_UNEXPECTED;
+  }
+
   NS_ENSURE_ARG(aPrefName);
   const char *pref = getPrefName(aPrefName);
   return PREF_GetIntPref(pref, _retval, mIsDefault);
@@ -213,6 +243,10 @@ NS_IMETHODIMP nsPrefBranch::GetIntPref(const char *aPrefName, PRInt32 *_retval)
 
 NS_IMETHODIMP nsPrefBranch::SetIntPref(const char *aPrefName, PRInt32 aValue)
 {
+  if (NS_UNLIKELY(!nsPrefService::CheckAndLogBackgroundThreadUse())) {
+    return NS_ERROR_UNEXPECTED;
+  }
+
 #ifdef MOZ_IPC
   if (GetContentChild()) {
     NS_ERROR("cannot set pref from content process");
@@ -227,6 +261,10 @@ NS_IMETHODIMP nsPrefBranch::SetIntPref(const char *aPrefName, PRInt32 aValue)
 
 NS_IMETHODIMP nsPrefBranch::GetComplexValue(const char *aPrefName, const nsIID & aType, void **_retval)
 {
+  if (NS_UNLIKELY(!nsPrefService::CheckAndLogBackgroundThreadUse())) {
+    return NS_ERROR_UNEXPECTED;
+  }
+
   NS_ENSURE_ARG(aPrefName);
 
   nsresult       rv;
@@ -388,6 +426,10 @@ NS_IMETHODIMP nsPrefBranch::GetComplexValue(const char *aPrefName, const nsIID &
 
 NS_IMETHODIMP nsPrefBranch::SetComplexValue(const char *aPrefName, const nsIID & aType, nsISupports *aValue)
 {
+  if (NS_UNLIKELY(!nsPrefService::CheckAndLogBackgroundThreadUse())) {
+    return NS_ERROR_UNEXPECTED;
+  }
+
 #ifdef MOZ_IPC
   if (GetContentChild()) {
     NS_ERROR("cannot set pref from content process");
@@ -479,6 +521,10 @@ NS_IMETHODIMP nsPrefBranch::SetComplexValue(const char *aPrefName, const nsIID &
 
 NS_IMETHODIMP nsPrefBranch::ClearUserPref(const char *aPrefName)
 {
+  if (NS_UNLIKELY(!nsPrefService::CheckAndLogBackgroundThreadUse())) {
+    return NS_ERROR_UNEXPECTED;
+  }
+
 #ifdef MOZ_IPC
   if (GetContentChild()) {
     NS_ERROR("cannot set pref from content process");
@@ -493,6 +539,10 @@ NS_IMETHODIMP nsPrefBranch::ClearUserPref(const char *aPrefName)
 
 NS_IMETHODIMP nsPrefBranch::PrefHasUserValue(const char *aPrefName, PRBool *_retval)
 {
+  if (NS_UNLIKELY(!nsPrefService::CheckAndLogBackgroundThreadUse())) {
+    return NS_ERROR_UNEXPECTED;
+  }
+
   NS_ENSURE_ARG_POINTER(_retval);
   NS_ENSURE_ARG(aPrefName);
   const char *pref = getPrefName(aPrefName);
@@ -502,6 +552,10 @@ NS_IMETHODIMP nsPrefBranch::PrefHasUserValue(const char *aPrefName, PRBool *_ret
 
 NS_IMETHODIMP nsPrefBranch::LockPref(const char *aPrefName)
 {
+  if (NS_UNLIKELY(!nsPrefService::CheckAndLogBackgroundThreadUse())) {
+    return NS_ERROR_UNEXPECTED;
+  }
+
 #ifdef MOZ_IPC
   if (GetContentChild()) {
     NS_ERROR("cannot lock pref from content process");
@@ -516,6 +570,10 @@ NS_IMETHODIMP nsPrefBranch::LockPref(const char *aPrefName)
 
 NS_IMETHODIMP nsPrefBranch::PrefIsLocked(const char *aPrefName, PRBool *_retval)
 {
+  if (NS_UNLIKELY(!nsPrefService::CheckAndLogBackgroundThreadUse())) {
+    return NS_ERROR_UNEXPECTED;
+  }
+
 #ifdef MOZ_IPC
   if (GetContentChild()) {
     NS_ERROR("cannot check lock pref from content process");
@@ -532,6 +590,10 @@ NS_IMETHODIMP nsPrefBranch::PrefIsLocked(const char *aPrefName, PRBool *_retval)
 
 NS_IMETHODIMP nsPrefBranch::UnlockPref(const char *aPrefName)
 {
+  if (NS_UNLIKELY(!nsPrefService::CheckAndLogBackgroundThreadUse())) {
+    return NS_ERROR_UNEXPECTED;
+  }
+
 #ifdef MOZ_IPC
   if (GetContentChild()) {
     NS_ERROR("cannot unlock pref from content process");
@@ -552,6 +614,10 @@ NS_IMETHODIMP nsPrefBranch::ResetBranch(const char *aStartingAt)
 
 NS_IMETHODIMP nsPrefBranch::DeleteBranch(const char *aStartingAt)
 {
+  if (NS_UNLIKELY(!nsPrefService::CheckAndLogBackgroundThreadUse())) {
+    return NS_ERROR_UNEXPECTED;
+  }
+
 #ifdef MOZ_IPC
   if (GetContentChild()) {
     NS_ERROR("cannot set pref from content process");
@@ -566,6 +632,10 @@ NS_IMETHODIMP nsPrefBranch::DeleteBranch(const char *aStartingAt)
 
 NS_IMETHODIMP nsPrefBranch::GetChildList(const char *aStartingAt, PRUint32 *aCount, char ***aChildArray)
 {
+  if (NS_UNLIKELY(!nsPrefService::CheckAndLogBackgroundThreadUse())) {
+    return NS_ERROR_UNEXPECTED;
+  }
+
   char            **outArray;
   PRInt32         numPrefs;
   PRInt32         dwIndex;
@@ -625,6 +695,10 @@ NS_IMETHODIMP nsPrefBranch::GetChildList(const char *aStartingAt, PRUint32 *aCou
 
 NS_IMETHODIMP nsPrefBranch::AddObserver(const char *aDomain, nsIObserver *aObserver, PRBool aHoldWeak)
 {
+  if (NS_UNLIKELY(!nsPrefService::CheckAndLogBackgroundThreadUse())) {
+    return NS_ERROR_UNEXPECTED;
+  }
+
   PrefCallback *pCallback;
   const char *pref;
 
@@ -670,6 +744,10 @@ NS_IMETHODIMP nsPrefBranch::AddObserver(const char *aDomain, nsIObserver *aObser
 
 NS_IMETHODIMP nsPrefBranch::RemoveObserver(const char *aDomain, nsIObserver *aObserver)
 {
+  if (NS_UNLIKELY(!nsPrefService::CheckAndLogBackgroundThreadUse())) {
+    return NS_ERROR_UNEXPECTED;
+  }
+
   NS_ENSURE_ARG(aDomain);
   NS_ENSURE_ARG(aObserver);
 
diff --git a/modules/libpref/src/nsPrefService.cpp b/modules/libpref/src/nsPrefService.cpp
index 760d3c5a110..05cc53ea389 100644
--- a/modules/libpref/src/nsPrefService.cpp
+++ b/modules/libpref/src/nsPrefService.cpp
@@ -46,6 +46,7 @@
 #include "nsAppDirectoryServiceDefs.h"
 #include "nsDirectoryServiceDefs.h"
 #include "nsICategoryManager.h"
+#include "nsIConsoleService.h"
 #include "nsCategoryManagerUtils.h"
 #include "nsNetUtil.h"
 #include "nsIFile.h"
@@ -60,6 +61,7 @@
 #include "nsCOMArray.h"
 #include "nsXPCOMCID.h"
 #include "nsAutoPtr.h"
+#include "nsThreadUtils.h"
 
 #include "nsQuickSort.h"
 #include "prmem.h"
@@ -219,6 +221,10 @@ NS_IMETHODIMP nsPrefService::Observe(nsISupports *aSubject, const char *aTopic,
 
 NS_IMETHODIMP nsPrefService::ReadUserPrefs(nsIFile *aFile)
 {
+  if (NS_UNLIKELY(!CheckAndLogBackgroundThreadUse())) {
+    return NS_ERROR_UNEXPECTED;
+  }
+
 #ifdef MOZ_IPC
   if (XRE_GetProcessType() == GeckoProcessType_Content) {
     NS_ERROR("cannot load prefs from content process");
@@ -242,6 +248,10 @@ NS_IMETHODIMP nsPrefService::ReadUserPrefs(nsIFile *aFile)
 
 NS_IMETHODIMP nsPrefService::ResetPrefs()
 {
+  if (NS_UNLIKELY(!CheckAndLogBackgroundThreadUse())) {
+    return NS_ERROR_UNEXPECTED;
+  }
+
 #ifdef MOZ_IPC
   if (XRE_GetProcessType() == GeckoProcessType_Content) {
     NS_ERROR("cannot set prefs from content process");
@@ -260,6 +270,10 @@ NS_IMETHODIMP nsPrefService::ResetPrefs()
 
 NS_IMETHODIMP nsPrefService::ResetUserPrefs()
 {
+  if (NS_UNLIKELY(!CheckAndLogBackgroundThreadUse())) {
+    return NS_ERROR_UNEXPECTED;
+  }
+
 #ifdef MOZ_IPC
   if (XRE_GetProcessType() == GeckoProcessType_Content) {
     NS_ERROR("cannot set prefs from content process");
@@ -273,6 +287,10 @@ NS_IMETHODIMP nsPrefService::ResetUserPrefs()
 
 NS_IMETHODIMP nsPrefService::SavePrefFile(nsIFile *aFile)
 {
+  if (NS_UNLIKELY(!CheckAndLogBackgroundThreadUse())) {
+    return NS_ERROR_UNEXPECTED;
+  }
+
 #ifdef MOZ_IPC
   if (XRE_GetProcessType() == GeckoProcessType_Content) {
     NS_ERROR("cannot save prefs from content process");
@@ -361,6 +379,10 @@ NS_IMETHODIMP nsPrefService::MirrorPreferences(nsTArray<PrefTuple, nsTArrayInfal
 
 NS_IMETHODIMP nsPrefService::GetBranch(const char *aPrefRoot, nsIPrefBranch **_retval)
 {
+  if (NS_UNLIKELY(!CheckAndLogBackgroundThreadUse())) {
+    return NS_ERROR_UNEXPECTED;
+  }
+
   nsresult rv;
 
   if ((nsnull != aPrefRoot) && (*aPrefRoot != '\0')) {
@@ -379,6 +401,10 @@ NS_IMETHODIMP nsPrefService::GetBranch(const char *aPrefRoot, nsIPrefBranch **_r
 
 NS_IMETHODIMP nsPrefService::GetDefaultBranch(const char *aPrefRoot, nsIPrefBranch **_retval)
 {
+  if (NS_UNLIKELY(!CheckAndLogBackgroundThreadUse())) {
+    return NS_ERROR_UNEXPECTED;
+  }
+
   nsresult rv;
 
   // TODO: - cache this stuff and allow consumers to share branches (hold weak references I think)
@@ -933,3 +959,22 @@ static nsresult pref_InitInitialObjects()
 
   return pref_LoadPrefsInDirList(NS_EXT_PREFS_DEFAULTS_DIR_LIST);
 }
+
+// static
+bool
+nsPrefService::CheckAndLogBackgroundThreadUse()
+{
+  if (NS_IsMainThread()) {
+    return true;
+  }
+
+  NS_WARNING("Cannot be used on a background thread!");
+  nsCOMPtr<nsIConsoleService> cs =
+    do_GetService("@mozilla.org/consoleservice;1");
+  if (cs) {
+    (void)cs->LogStringMessage(NS_LITERAL_STRING(
+      "Invalid use of the preferences on a background thread!"
+    ).get());
+  }
+  return false;
+}
diff --git a/modules/libpref/src/nsPrefService.h b/modules/libpref/src/nsPrefService.h
index 9cab286bfac..e82b2e381a2 100644
--- a/modules/libpref/src/nsPrefService.h
+++ b/modules/libpref/src/nsPrefService.h
@@ -67,7 +67,13 @@ public:
   virtual ~nsPrefService();
 
   nsresult Init();
-                           
+
+  /**
+   * Checks to make sure we are on the main thread and log if we are not.
+   *
+   * @returns true if on the main thread, and false otherwise.
+   */
+  static bool CheckAndLogBackgroundThreadUse();
 protected:
   nsresult NotifyServiceObservers(const char *aSubject);
   nsresult UseDefaultPrefFile();