diff --git a/chrome/nsChromeRegistry.cpp b/chrome/nsChromeRegistry.cpp
index 2a9be41f0ae5..74e16f8175f8 100644
--- a/chrome/nsChromeRegistry.cpp
+++ b/chrome/nsChromeRegistry.cpp
@@ -656,7 +656,7 @@ nsChromeRegistry::GetDirectionForLocale(const nsACString& aLocale)
     return false;
   }
 
-  nsXPIDLCString dir;
+  nsCString dir;
   prefBranch->GetCharPref(prefString.get(), getter_Copies(dir));
   if (dir.IsEmpty()) {
     int32_t hyphen = prefString.FindChar('-');
diff --git a/chrome/nsChromeRegistryChrome.cpp b/chrome/nsChromeRegistryChrome.cpp
index 16bd9e9c1fe4..8df9079d1cbd 100644
--- a/chrome/nsChromeRegistryChrome.cpp
+++ b/chrome/nsChromeRegistryChrome.cpp
@@ -128,7 +128,7 @@ nsChromeRegistryChrome::Init()
   if (!prefs) {
     NS_WARNING("Could not get pref service!");
   } else {
-    nsXPIDLCString provider;
+    nsCString provider;
     rv = prefs->GetCharPref(SELECTED_SKIN_PREF, getter_Copies(provider));
     if (NS_SUCCEEDED(rv))
       mSelectedSkin = provider;
@@ -275,7 +275,7 @@ nsChromeRegistryChrome::Observe(nsISupports *aSubject, const char *aTopic,
     NS_ConvertUTF16toUTF8 pref(someData);
 
     if (pref.EqualsLiteral(SELECTED_SKIN_PREF)) {
-      nsXPIDLCString provider;
+      nsCString provider;
       rv = prefs->GetCharPref(pref.get(), getter_Copies(provider));
       if (NS_FAILED(rv)) {
         NS_ERROR("Couldn't get new skin pref!");
diff --git a/dom/base/nsContentAreaDragDrop.cpp b/dom/base/nsContentAreaDragDrop.cpp
index d80e10d4994a..dcf1d3dde73b 100644
--- a/dom/base/nsContentAreaDragDrop.cpp
+++ b/dom/base/nsContentAreaDragDrop.cpp
@@ -580,7 +580,7 @@ DragDataProducer::Produce(DataTransfer* aDataTransfer,
             nsAutoCString extension;
             imgUrl->GetFileExtension(extension);
 
-            nsXPIDLCString mimeType;
+            nsCString mimeType;
             imgRequest->GetMimeType(getter_Copies(mimeType));
 
             nsCOMPtr<nsIMIMEInfo> mimeInfo;
diff --git a/dom/base/nsCopySupport.cpp b/dom/base/nsCopySupport.cpp
index aa45753fe55b..8e51be74413c 100644
--- a/dom/base/nsCopySupport.cpp
+++ b/dom/base/nsCopySupport.cpp
@@ -628,7 +628,7 @@ static nsresult AppendImagePromise(nsITransferable* aTransferable,
   rv = imgUrl->GetFileExtension(extension);
   NS_ENSURE_SUCCESS(rv, rv);
 
-  nsXPIDLCString mimeType;
+  nsCString mimeType;
   rv = aImgRequest->GetMimeType(getter_Copies(mimeType));
   NS_ENSURE_SUCCESS(rv, rv);
 
diff --git a/dom/base/nsGlobalWindowCommands.cpp b/dom/base/nsGlobalWindowCommands.cpp
index df1117f6cc34..c251e28c87de 100644
--- a/dom/base/nsGlobalWindowCommands.cpp
+++ b/dom/base/nsGlobalWindowCommands.cpp
@@ -783,7 +783,7 @@ nsClipboardGetContentsCommand::DoClipboardCommand(const char *aCommandName, nsIC
 
   nsAutoCString mimeType("text/plain");
 
-  nsXPIDLCString format;    // nsICommandParams needs to use nsACString
+  nsCString format;    // nsICommandParams needs to use nsACString
   if (NS_SUCCEEDED(aParams->GetCStringValue("format", getter_Copies(format))))
     mimeType.Assign(format);
 
diff --git a/dom/html/ImageDocument.cpp b/dom/html/ImageDocument.cpp
index 7d3f5267290f..d5c686c27b9f 100644
--- a/dom/html/ImageDocument.cpp
+++ b/dom/html/ImageDocument.cpp
@@ -777,13 +777,13 @@ ImageDocument::UpdateTitleAndCharset()
   }
 
   if (imageRequest) {
-    nsXPIDLCString mimeType;
+    nsCString mimeType;
     imageRequest->GetMimeType(getter_Copies(mimeType));
     ToUpperCase(mimeType);
-    nsXPIDLCString::const_iterator start, end;
+    nsCString::const_iterator start, end;
     mimeType.BeginReading(start);
     mimeType.EndReading(end);
-    nsXPIDLCString::const_iterator iter = end;
+    nsCString::const_iterator iter = end;
     if (FindInReadable(NS_LITERAL_CSTRING("IMAGE/"), start, iter) &&
         iter != end) {
       // strip out "X-" if any
diff --git a/dom/html/nsHTMLDocument.cpp b/dom/html/nsHTMLDocument.cpp
index 4afeb644e6fc..0e1e2a057ec8 100644
--- a/dom/html/nsHTMLDocument.cpp
+++ b/dom/html/nsHTMLDocument.cpp
@@ -1371,7 +1371,7 @@ nsHTMLDocument::GetCookie(nsAString& aCookie, ErrorResult& rv)
       }
     }
 
-    nsXPIDLCString cookie;
+    nsCString cookie;
     service->GetCookieString(codebaseURI, channel, getter_Copies(cookie));
     // CopyUTF8toUTF16 doesn't handle error
     // because it assumes that the input is valid.
@@ -3668,7 +3668,7 @@ nsHTMLDocument::QueryCommandValue(const nsAString& commandID,
   // aValue will wind up being the empty string.  This is fine -- we want to
   // return "" in that case anyway (bug 738385), so we just return NS_OK
   // regardless.
-  nsXPIDLCString cStringResult;
+  nsCString cStringResult;
   cmdParams->GetCStringValue("state_attribute",
                              getter_Copies(cStringResult));
   CopyUTF8toUTF16(cStringResult, aValue);
diff --git a/dom/ipc/ContentChild.cpp b/dom/ipc/ContentChild.cpp
index 6dfc0a8680bf..7d5408327a79 100644
--- a/dom/ipc/ContentChild.cpp
+++ b/dom/ipc/ContentChild.cpp
@@ -432,7 +432,7 @@ ConsoleListener::Observe(nsIConsoleMessage* aMessage)
   nsCOMPtr<nsIScriptError> scriptError = do_QueryInterface(aMessage);
   if (scriptError) {
     nsAutoString msg, sourceName, sourceLine;
-    nsXPIDLCString category;
+    nsCString category;
     uint32_t lineNum, colNum, flags;
 
     nsresult rv = scriptError->GetErrorMessage(msg);
diff --git a/dom/svg/SVGDocument.cpp b/dom/svg/SVGDocument.cpp
index 272dd77e6f8a..5e4079f22157 100644
--- a/dom/svg/SVGDocument.cpp
+++ b/dom/svg/SVGDocument.cpp
@@ -115,7 +115,7 @@ SVGDocument::EnsureNonSVGUserAgentStyleSheetsLoaded()
           nsAutoCString name;
           icStr->GetData(name);
 
-          nsXPIDLCString spec;
+          nsCString spec;
           catMan->GetCategoryEntry("agent-style-sheets", name.get(),
                                    getter_Copies(spec));
 
diff --git a/dom/xslt/xslt/txStylesheetCompiler.cpp b/dom/xslt/xslt/txStylesheetCompiler.cpp
index 508d4b41b701..d4d3e02d4742 100644
--- a/dom/xslt/xslt/txStylesheetCompiler.cpp
+++ b/dom/xslt/xslt/txStylesheetCompiler.cpp
@@ -1042,7 +1042,7 @@ findFunction(nsIAtom* aName, int32_t aNamespaceID,
         rv = txNamespaceManager::getNamespaceURI(aNamespaceID, namespaceURI);
         NS_ENSURE_SUCCESS(rv, rv);
 
-        nsXPIDLCString contractID;
+        nsCString contractID;
         rv = catman->GetCategoryEntry("XSLT-extension-functions",
                                       NS_ConvertUTF16toUTF8(namespaceURI).get(),
                                       getter_Copies(contractID));
diff --git a/editor/composer/nsComposerCommands.cpp b/editor/composer/nsComposerCommands.cpp
index be3e34b3b80c..723a3e756e33 100644
--- a/editor/composer/nsComposerCommands.cpp
+++ b/editor/composer/nsComposerCommands.cpp
@@ -627,7 +627,7 @@ nsMultiStateCommand::DoCommandParams(const char *aCommandName,
 
   nsAutoString tString;
   if (aParams) {
-    nsXPIDLCString s;
+    nsCString s;
     nsresult rv = aParams->GetCStringValue(STATE_ATTRIBUTE, getter_Copies(s));
     if (NS_SUCCEEDED(rv))
       CopyASCIItoUTF16(s, tString);
@@ -1508,7 +1508,7 @@ nsInsertTagCommand::DoCommandParams(const char *aCommandName,
   }
 
   // do we have an href to use for creating link?
-  nsXPIDLCString s;
+  nsCString s;
   nsresult rv = aParams->GetCStringValue(STATE_ATTRIBUTE, getter_Copies(s));
   NS_ENSURE_SUCCESS(rv, rv);
   nsAutoString attrib;
diff --git a/editor/composer/nsComposerDocumentCommands.cpp b/editor/composer/nsComposerDocumentCommands.cpp
index 7ceb780f771d..a316afbe36d6 100644
--- a/editor/composer/nsComposerDocumentCommands.cpp
+++ b/editor/composer/nsComposerDocumentCommands.cpp
@@ -275,7 +275,7 @@ nsSetDocumentStateCommand::DoCommandParams(const char *aCommandName,
       return NS_ERROR_INVALID_ARG;
     }
 
-    nsXPIDLCString newValue;
+    nsCString newValue;
     nsresult rv = aParams->GetCStringValue(STATE_ATTRIBUTE,
                                            getter_Copies(newValue));
     if (NS_WARN_IF(NS_FAILED(rv))) {
diff --git a/extensions/auth/nsAuthGSSAPI.cpp b/extensions/auth/nsAuthGSSAPI.cpp
index ba4fcb0a3c01..1a9a7763f903 100644
--- a/extensions/auth/nsAuthGSSAPI.cpp
+++ b/extensions/auth/nsAuthGSSAPI.cpp
@@ -102,7 +102,7 @@ static PRFuncPtr KLCacheHasValidTicketsPtr;
 static nsresult
 gssInit()
 {
-    nsXPIDLCString libPath;
+    nsCString libPath;
     nsCOMPtr<nsIPrefBranch> prefs = do_GetService(NS_PREFSERVICE_CONTRACTID);
     if (prefs) {
         prefs->GetCharPref(kNegotiateAuthGssLib, getter_Copies(libPath));
diff --git a/extensions/pref/autoconfig/src/nsAutoConfig.cpp b/extensions/pref/autoconfig/src/nsAutoConfig.cpp
index b078adff40d7..d7d5f7436bf5 100644
--- a/extensions/pref/autoconfig/src/nsAutoConfig.cpp
+++ b/extensions/pref/autoconfig/src/nsAutoConfig.cpp
@@ -208,7 +208,6 @@ nsresult nsAutoConfig::downloadAutoConfig()
 {
     nsresult rv;
     nsAutoCString emailAddr;
-    nsXPIDLCString urlName;
     static bool firstTime = true;
 
     if (mConfigURL.IsEmpty()) {
@@ -463,7 +462,7 @@ nsresult nsAutoConfig::getEmailAddr(nsACString & emailAddr)
 {
 
     nsresult rv;
-    nsXPIDLCString prefValue;
+    nsCString prefValue;
 
     /* Getting an email address through set of three preferences:
        First getting a default account with
diff --git a/intl/strres/nsStringBundle.cpp b/intl/strres/nsStringBundle.cpp
index 7fb2ce31b17e..7dcfc729f41e 100644
--- a/intl/strres/nsStringBundle.cpp
+++ b/intl/strres/nsStringBundle.cpp
@@ -643,7 +643,6 @@ nsStringBundleService::FormatWithBundle(nsIStringBundle* bundle, nsresult aStatu
                                         nsAString& result)
 {
   nsresult rv;
-  nsXPIDLCString key;
 
   // try looking up the error message with the int key:
   uint16_t code = NS_ERROR_GET_CODE(aStatus);
diff --git a/js/xpconnect/src/XPCWrappedNativeInfo.cpp b/js/xpconnect/src/XPCWrappedNativeInfo.cpp
index 1cd3a2a0c0ad..4a3690cdae78 100644
--- a/js/xpconnect/src/XPCWrappedNativeInfo.cpp
+++ b/js/xpconnect/src/XPCWrappedNativeInfo.cpp
@@ -56,7 +56,7 @@ XPCNativeMember::Resolve(XPCCallContext& ccx, XPCNativeInterface* iface,
     MOZ_ASSERT(iface == GetInterface());
     if (IsConstant()) {
         RootedValue resultVal(ccx);
-        nsXPIDLCString name;
+        nsCString name;
         if (NS_FAILED(iface->GetInterfaceInfo()->GetConstant(mIndex, &resultVal,
                                                              getter_Copies(name))))
             return false;
diff --git a/layout/base/nsStyleSheetService.cpp b/layout/base/nsStyleSheetService.cpp
index a2ccb87d8a50..60d8b1d92127 100644
--- a/layout/base/nsStyleSheetService.cpp
+++ b/layout/base/nsStyleSheetService.cpp
@@ -73,7 +73,7 @@ nsStyleSheetService::RegisterFromEnumerator(nsICategoryManager  *aManager,
     nsAutoCString name;
     icStr->GetData(name);
 
-    nsXPIDLCString spec;
+    nsCString spec;
     aManager->GetCategoryEntry(aCategory, name.get(), getter_Copies(spec));
 
     nsCOMPtr<nsIURI> uri;
diff --git a/modules/libpref/Preferences.cpp b/modules/libpref/Preferences.cpp
index c7cb8f046815..017a0b3bd735 100644
--- a/modules/libpref/Preferences.cpp
+++ b/modules/libpref/Preferences.cpp
@@ -754,7 +754,7 @@ Preferences::Init()
     return NS_OK;
   }
 
-  nsXPIDLCString lockFileName;
+  nsCString lockFileName;
   /*
    * The following is a small hack which will allow us to only load the library
    * which supports the netscape.cfg file if the preference is defined. We
diff --git a/modules/libpref/nsPrefBranch.cpp b/modules/libpref/nsPrefBranch.cpp
index d90a9bc64a6a..22885b30b66f 100644
--- a/modules/libpref/nsPrefBranch.cpp
+++ b/modules/libpref/nsPrefBranch.cpp
@@ -242,7 +242,7 @@ NS_IMETHODIMP nsPrefBranch::GetStringPref(const char *aPrefName,
                                           uint8_t _argc,
                                           nsACString& _retval)
 {
-  nsXPIDLCString utf8String;
+  nsCString utf8String;
   nsresult rv = GetCharPref(aPrefName, getter_Copies(utf8String));
   if (NS_SUCCEEDED(rv)) {
     _retval = utf8String;
@@ -300,8 +300,8 @@ NS_IMETHODIMP nsPrefBranch::GetComplexValue(const char *aPrefName, const nsIID &
 {
   NS_ENSURE_ARG(aPrefName);
 
-  nsresult       rv;
-  nsXPIDLCString utf8String;
+  nsresult rv;
+  nsCString utf8String;
 
   // we have to do this one first because it's different than all the rest
   if (aType.Equals(NS_GET_IID(nsIPrefLocalizedString))) {
diff --git a/netwerk/base/nsChannelClassifier.cpp b/netwerk/base/nsChannelClassifier.cpp
index 3a61db1bda36..d353509325f2 100644
--- a/netwerk/base/nsChannelClassifier.cpp
+++ b/netwerk/base/nsChannelClassifier.cpp
@@ -744,7 +744,7 @@ nsChannelClassifier::HasBeenClassified(nsIChannel *aChannel)
         return false;
     }
 
-    nsXPIDLCString tag;
+    nsCString tag;
     cacheEntry->GetMetaDataElement("necko:classified", getter_Copies(tag));
     return tag.EqualsLiteral("1");
 }
diff --git a/netwerk/base/nsProtocolProxyService.cpp b/netwerk/base/nsProtocolProxyService.cpp
index 59bf5d0518d5..1fd3103b0dbb 100644
--- a/netwerk/base/nsProtocolProxyService.cpp
+++ b/netwerk/base/nsProtocolProxyService.cpp
@@ -428,7 +428,7 @@ proxy_GetStringPref(nsIPrefBranch *aPrefBranch,
                     const char    *aPref,
                     nsCString     &aResult)
 {
-    nsXPIDLCString temp;
+    nsCString temp;
     nsresult rv = aPrefBranch->GetCharPref(aPref, getter_Copies(temp));
     if (NS_FAILED(rv))
         aResult.Truncate();
@@ -561,7 +561,7 @@ nsProtocolProxyService::ReloadNetworkPAC()
     }
 
     if (type == PROXYCONFIG_PAC) {
-        nsXPIDLCString pacSpec;
+        nsCString pacSpec;
         prefs->GetCharPref(PROXY_PREF("autoconfig_url"),
                            getter_Copies(pacSpec));
         if (!pacSpec.IsEmpty()) {
@@ -693,7 +693,7 @@ nsProtocolProxyService::PrefsChanged(nsIPrefBranch *prefBranch,
 {
     nsresult rv = NS_OK;
     bool reloadPAC = false;
-    nsXPIDLCString tempString;
+    nsCString tempString;
 
     if (!pref || !strcmp(pref, PROXY_PREF("type"))) {
         int32_t type = -1;
@@ -1247,7 +1247,7 @@ nsProtocolProxyService::ReloadPAC()
     if (NS_FAILED(rv))
         return NS_OK;
 
-    nsXPIDLCString pacSpec;
+    nsCString pacSpec;
     if (type == PROXYCONFIG_PAC)
         prefs->GetCharPref(PROXY_PREF("autoconfig_url"), getter_Copies(pacSpec));
     else if (type == PROXYCONFIG_WPAD)
diff --git a/netwerk/cache2/OldWrappers.cpp b/netwerk/cache2/OldWrappers.cpp
index 6b583894aa67..66d16a8935c4 100644
--- a/netwerk/cache2/OldWrappers.cpp
+++ b/netwerk/cache2/OldWrappers.cpp
@@ -207,7 +207,7 @@ NS_IMETHODIMP _OldVisitCallbackWrapper::VisitEntry(const char * deviceID,
   *_retval = true;
 
   // Read all informative properties from the entry.
-  nsXPIDLCString clientId;
+  nsCString clientId;
   rv = entryInfo->GetClientID(getter_Copies(clientId));
   if (NS_FAILED(rv))
     return NS_OK;
diff --git a/netwerk/dns/nsIDNService.cpp b/netwerk/dns/nsIDNService.cpp
index 9076c85db523..9cc8fdcf6fa1 100644
--- a/netwerk/dns/nsIDNService.cpp
+++ b/netwerk/dns/nsIDNService.cpp
@@ -124,7 +124,7 @@ void nsIDNService::prefsChanged(nsIPrefBranch *prefBranch, const char16_t *pref)
       mIDNUseWhitelist = val;
   }
   if (!pref || NS_LITERAL_STRING(NS_NET_PREF_IDNRESTRICTION).Equals(pref)) {
-    nsXPIDLCString profile;
+    nsCString profile;
     if (NS_FAILED(prefBranch->GetCharPref(NS_NET_PREF_IDNRESTRICTION,
                                           getter_Copies(profile)))) {
       profile.Truncate();
diff --git a/netwerk/mime/nsMIMEHeaderParamImpl.cpp b/netwerk/mime/nsMIMEHeaderParamImpl.cpp
index 5cacfc9ecde5..49eeae025f70 100644
--- a/netwerk/mime/nsMIMEHeaderParamImpl.cpp
+++ b/netwerk/mime/nsMIMEHeaderParamImpl.cpp
@@ -78,8 +78,8 @@ nsMIMEHeaderParamImpl::DoGetParameter(const nsACString& aHeaderVal,
 
     // get parameter (decode RFC 2231/5987 when applicable, as specified by
     // aDecoding (5987 being a subset of 2231) and return charset.)
-    nsXPIDLCString med;
-    nsXPIDLCString charset;
+    nsCString med;
+    nsCString charset;
     rv = DoParameterInternal(PromiseFlatCString(aHeaderVal).get(), aParamName,
                              aDecoding, getter_Copies(charset), aLang,
                              getter_Copies(med));
diff --git a/netwerk/protocol/http/Http2Session.cpp b/netwerk/protocol/http/Http2Session.cpp
index c6e782ead1ae..0a65b643db29 100644
--- a/netwerk/protocol/http/Http2Session.cpp
+++ b/netwerk/protocol/http/Http2Session.cpp
@@ -1924,7 +1924,7 @@ Http2Session::CachePushCheckCallback::OnCacheEntryCheck(nsICacheEntry *entry, ns
   }
 
   // Get the method that was used to generate the cached response
-  nsXPIDLCString buf;
+  nsCString buf;
   rv = entry->GetMetaDataElement("request-method", getter_Copies(buf));
   if (NS_FAILED(rv)) {
     // Can't check request method, accept the push
@@ -1986,7 +1986,7 @@ Http2Session::CachePushCheckCallback::OnCacheEntryCheck(nsICacheEntry *entry, ns
     return NS_OK;
   }
 
-  nsXPIDLCString cachedAuth;
+  nsCString cachedAuth;
   rv = entry->GetMetaDataElement("auth", getter_Copies(cachedAuth));
   if (NS_SUCCEEDED(rv)) {
     uint32_t lastModifiedTime;
diff --git a/netwerk/protocol/http/HttpBaseChannel.cpp b/netwerk/protocol/http/HttpBaseChannel.cpp
index 547725da1d17..1456d17bf212 100644
--- a/netwerk/protocol/http/HttpBaseChannel.cpp
+++ b/netwerk/protocol/http/HttpBaseChannel.cpp
@@ -3201,7 +3201,7 @@ HttpBaseChannel::AddCookiesToRequest()
 
   bool useCookieService =
     (XRE_IsParentProcess());
-  nsXPIDLCString cookie;
+  nsCString cookie;
   if (useCookieService) {
     nsICookieService *cs = gHttpHandler->GetCookieService();
     if (cs) {
diff --git a/netwerk/protocol/http/nsHttp.cpp b/netwerk/protocol/http/nsHttp.cpp
index 96bf21583709..23ee0e7ed71a 100644
--- a/netwerk/protocol/http/nsHttp.cpp
+++ b/netwerk/protocol/http/nsHttp.cpp
@@ -473,7 +473,7 @@ ValidationRequired(bool isForcedValid, nsHttpResponseHead *cachedResponseHead,
 nsresult
 GetHttpResponseHeadFromCacheEntry(nsICacheEntry *entry, nsHttpResponseHead *cachedResponseHead)
 {
-    nsXPIDLCString buf;
+    nsCString buf;
     // A "original-response-headers" metadata element holds network original headers,
     // i.e. the headers in the form as they arrieved from the network.
     // We need to get the network original headers first, because we need to keep them
@@ -532,7 +532,7 @@ DetermineFramingAndImmutability(nsICacheEntry *entry,
         nsHttpResponseHead *responseHead, bool isHttps,
         bool *weaklyFramed, bool *isImmutable)
 {
-    nsXPIDLCString framedBuf;
+    nsCString framedBuf;
     nsresult rv = entry->GetMetaDataElement("strongly-framed", getter_Copies(framedBuf));
     // describe this in terms of explicitly weakly framed so as to be backwards
     // compatible with old cache contents which dont have strongly-framed makers
diff --git a/netwerk/protocol/http/nsHttpChannel.cpp b/netwerk/protocol/http/nsHttpChannel.cpp
index ab397d186cb5..d473d047c09f 100644
--- a/netwerk/protocol/http/nsHttpChannel.cpp
+++ b/netwerk/protocol/http/nsHttpChannel.cpp
@@ -3166,7 +3166,7 @@ nsHttpChannel::ResponseWouldVary(nsICacheEntry* entry)
 
             // check the last value of the given request header to see if it has
             // since changed.  if so, then indeed the cached response is invalid.
-            nsXPIDLCString lastVal;
+            nsCString lastVal;
             entry->GetMetaDataElement(metaKey.get(), getter_Copies(lastVal));
             LOG(("nsHttpChannel::ResponseWouldVary [channel=%p] "
                      "stored value = \"%s\"\n",
@@ -4044,7 +4044,7 @@ nsHttpChannel::OnCacheEntryCheck(nsICacheEntry* entry, nsIApplicationCache* appC
     *aResult = ENTRY_WANTED;
     mCachedContentIsValid = false;
 
-    nsXPIDLCString buf;
+    nsCString buf;
 
     // Get the method that was used to generate the cached response
     rv = entry->GetMetaDataElement("request-method", getter_Copies(buf));
@@ -7859,7 +7859,7 @@ nsHttpChannel::GetCacheTokenCachedCharset(nsACString &_retval)
     if (!mCacheEntry)
         return NS_ERROR_NOT_AVAILABLE;
 
-    nsXPIDLCString cachedCharset;
+    nsCString cachedCharset;
     rv = mCacheEntry->GetMetaDataElement("charset",
                                          getter_Copies(cachedCharset));
     if (NS_SUCCEEDED(rv))
diff --git a/netwerk/protocol/http/nsHttpChannelAuthProvider.cpp b/netwerk/protocol/http/nsHttpChannelAuthProvider.cpp
index 9f34fc4a29ce..61311788ccbd 100644
--- a/netwerk/protocol/http/nsHttpChannelAuthProvider.cpp
+++ b/netwerk/protocol/http/nsHttpChannelAuthProvider.cpp
@@ -952,7 +952,7 @@ nsHttpChannelAuthProvider::GetCredentialsForChallenge(const char *challenge,
     // that we don't transparently authenticate as someone they're not
     // expecting to authenticate as.
     //
-    nsXPIDLCString result;
+    nsCString result;
     rv = GenCredsAndSetEntry(auth, proxyAuth, scheme.get(), host, port,
                              path.get(), realm.get(), challenge, *ident,
                              sessionStateGrip, getter_Copies(result));
@@ -1353,7 +1353,7 @@ NS_IMETHODIMP nsHttpChannelAuthProvider::OnAuthAvailable(nsISupports *aContext,
         return NS_OK;
     }
 
-    nsXPIDLCString creds;
+    nsCString creds;
     rv = GenCredsAndSetEntry(auth, mProxyAuth,
                              scheme.get(), host, port, path.get(),
                              realm.get(), mCurrentChallenge.get(), *ident,
@@ -1693,7 +1693,7 @@ nsHttpChannelAuthProvider::SetAuthorizationHeader(nsHttpAuthCache    *authCache,
         else
             identFromURI = true;
 
-        nsXPIDLCString temp;
+        nsCString temp;  // this must have the same lifetime as creds
         const char *creds     = entry->Creds();
         const char *challenge = entry->Challenge();
         // we can only send a preemptive Authorization header if we have either
diff --git a/netwerk/protocol/wyciwyg/nsWyciwygChannel.cpp b/netwerk/protocol/wyciwyg/nsWyciwygChannel.cpp
index baabb16872f2..879fef838272 100644
--- a/netwerk/protocol/wyciwyg/nsWyciwygChannel.cpp
+++ b/netwerk/protocol/wyciwyg/nsWyciwygChannel.cpp
@@ -533,14 +533,14 @@ nsWyciwygChannel::GetCharsetAndSource(int32_t* aSource, nsACString& aCharset)
     return NS_ERROR_NOT_AVAILABLE;
   }
 
-  nsXPIDLCString data;
+  nsCString data;
   mCacheEntry->GetMetaDataElement("charset", getter_Copies(data));
 
   if (data.IsEmpty()) {
     return NS_ERROR_NOT_AVAILABLE;
   }
 
-  nsXPIDLCString sourceStr;
+  nsCString sourceStr;
   mCacheEntry->GetMetaDataElement("charset-source", getter_Copies(sourceStr));
 
   int32_t source;
diff --git a/security/manager/ssl/nsNSSCertificate.cpp b/security/manager/ssl/nsNSSCertificate.cpp
index df35ff3f0893..5c24e1250f54 100644
--- a/security/manager/ssl/nsNSSCertificate.cpp
+++ b/security/manager/ssl/nsNSSCertificate.cpp
@@ -1512,7 +1512,7 @@ nsNSSCertificate::Read(nsIObjectInputStream* aStream)
     return rv;
   }
 
-  nsXPIDLCString str;
+  nsCString str;
   rv = aStream->ReadBytes(len, getter_Copies(str));
   if (NS_FAILED(rv)) {
     return rv;
diff --git a/toolkit/components/typeaheadfind/nsTypeAheadFind.cpp b/toolkit/components/typeaheadfind/nsTypeAheadFind.cpp
index 9139a5d1908c..aa336c227199 100644
--- a/toolkit/components/typeaheadfind/nsTypeAheadFind.cpp
+++ b/toolkit/components/typeaheadfind/nsTypeAheadFind.cpp
@@ -149,7 +149,7 @@ nsTypeAheadFind::PrefsReset()
   bool isSoundEnabled = true;
   prefBranch->GetBoolPref("accessibility.typeaheadfind.enablesound",
                            &isSoundEnabled);
-  nsXPIDLCString soundStr;
+  nsCString soundStr;
   if (isSoundEnabled)
     prefBranch->GetCharPref("accessibility.typeaheadfind.soundURL", getter_Copies(soundStr));
 
diff --git a/toolkit/components/url-classifier/nsUrlClassifierUtils.cpp b/toolkit/components/url-classifier/nsUrlClassifierUtils.cpp
index 8673ef8b916f..19aac73874e1 100644
--- a/toolkit/components/url-classifier/nsUrlClassifierUtils.cpp
+++ b/toolkit/components/url-classifier/nsUrlClassifierUtils.cpp
@@ -131,7 +131,7 @@ CreateClientInfo()
   nsCOMPtr<nsIPrefBranch> prefBranch =
     do_GetService(NS_PREFSERVICE_CONTRACTID);
 
-  nsXPIDLCString clientId;
+  nsCString clientId;
   nsresult rv = prefBranch->GetCharPref("browser.safebrowsing.id",
                                         getter_Copies(clientId));
 
@@ -552,7 +552,7 @@ nsUrlClassifierUtils::ReadProvidersFromPrefs(ProviderDictType& aDict)
     nsCString provider(entry->GetKey());
     nsPrintfCString owninListsPref("%s.lists", provider.get());
 
-    nsXPIDLCString owningLists;
+    nsCString owningLists;
     nsresult rv = prefBranch->GetCharPref(owninListsPref.get(),
                                           getter_Copies(owningLists));
     if (NS_FAILED(rv)) {
diff --git a/toolkit/xre/nsAppRunner.cpp b/toolkit/xre/nsAppRunner.cpp
index fac50695bc0d..a01f66ff8171 100644
--- a/toolkit/xre/nsAppRunner.cpp
+++ b/toolkit/xre/nsAppRunner.cpp
@@ -1797,7 +1797,7 @@ StartRemoteClient(const char* aDesktopStartupID,
   if (NS_FAILED(rv))
     return REMOTE_NOT_FOUND;
 
-  nsXPIDLCString response;
+  nsCString response;
   bool success = false;
   rv = client.SendCommandLine(program.get(), username, profile,
                               gArgc, gArgv, aDesktopStartupID,
@@ -4324,7 +4324,7 @@ XREMain::XRE_mainRun()
     rv = prefs->GetDefaultBranch(nullptr, getter_AddRefs(defaultPrefBranch));
 
     if (NS_SUCCEEDED(rv)) {
-      nsXPIDLCString sval;
+      nsCString sval;
       rv = defaultPrefBranch->GetCharPref("app.update.channel", getter_Copies(sval));
       if (NS_SUCCEEDED(rv)) {
         CrashReporter::AnnotateCrashReport(NS_LITERAL_CSTRING("ReleaseChannel"),
diff --git a/uriloader/base/nsURILoader.cpp b/uriloader/base/nsURILoader.cpp
index 616f61d877c0..bc8dcd80259d 100644
--- a/uriloader/base/nsURILoader.cpp
+++ b/uriloader/base/nsURILoader.cpp
@@ -412,9 +412,6 @@ nsresult nsDocumentOpenInfo::DispatchContent(nsIRequest *request, nsISupports *
 
   LOG(("  forceExternalHandling: %s", forceExternalHandling ? "yes" : "no"));
 
-  // The type or data the contentListener wants.
-  nsXPIDLCString desiredContentType;
-
   if (!forceExternalHandling)
   {
     //
@@ -690,7 +687,7 @@ nsDocumentOpenInfo::TryContentListener(nsIURIContentListener* aListener,
   NS_PRECONDITION(aChannel, "Must have a channel");
   
   bool listenerWantsContent = false;
-  nsXPIDLCString typeToUse;
+  nsCString typeToUse;
   
   if (mFlags & nsIURILoader::IS_CONTENT_PREFERRED) {
     aListener->IsPreferred(mContentType.get(),
diff --git a/uriloader/exthandler/nsExternalHelperAppService.cpp b/uriloader/exthandler/nsExternalHelperAppService.cpp
index a3554d421206..3b830b42c9fb 100644
--- a/uriloader/exthandler/nsExternalHelperAppService.cpp
+++ b/uriloader/exthandler/nsExternalHelperAppService.cpp
@@ -2737,7 +2737,7 @@ nsExternalHelperAppService::GetTypeFromExtension(const nsACString& aFileExt,
     nsAutoCString lowercaseFileExt(aFileExt);
     ToLowerCase(lowercaseFileExt);
     // Read the MIME type from the category entry, if available
-    nsXPIDLCString type;
+    nsCString type;
     nsresult rv = catMan->GetCategoryEntry("ext-to-type-mapping",
                                            lowercaseFileExt.get(),
                                            getter_Copies(type));
diff --git a/widget/cocoa/nsClipboard.mm b/widget/cocoa/nsClipboard.mm
index 137a54dd7296..251a190a69f3 100644
--- a/widget/cocoa/nsClipboard.mm
+++ b/widget/cocoa/nsClipboard.mm
@@ -444,7 +444,7 @@ nsClipboard::HasDataMatchingFlavors(const char** aFlavorList, uint32_t aLength,
             do_QueryElementAt(transferableFlavorList, j);
         if (!currentTransferableFlavor)
           continue;
-        nsXPIDLCString transferableFlavorStr;
+        nsCString transferableFlavorStr;
         currentTransferableFlavor->ToString(getter_Copies(transferableFlavorStr));
 
         for (uint32_t k = 0; k < aLength; k++) {
diff --git a/widget/cocoa/nsDragService.mm b/widget/cocoa/nsDragService.mm
index 91fd5e7e374d..ff66172d08c5 100644
--- a/widget/cocoa/nsDragService.mm
+++ b/widget/cocoa/nsDragService.mm
@@ -599,7 +599,7 @@ nsDragService::IsDataFlavorSupported(const char *aDataFlavor, bool *_retval)
         nsCOMPtr<nsISupportsCString> currentFlavor = do_QueryElementAt(flavorList, j);
         if (!currentFlavor)
           continue;
-        nsXPIDLCString flavorStr;
+        nsCString flavorStr;
         currentFlavor->ToString(getter_Copies(flavorStr));
         if (dataFlavor.Equals(flavorStr)) {
           *_retval = true;
diff --git a/xpfe/components/directory/nsDirectoryViewer.cpp b/xpfe/components/directory/nsDirectoryViewer.cpp
index dd2e5be33f55..d3eff5f0fe40 100644
--- a/xpfe/components/directory/nsDirectoryViewer.cpp
+++ b/xpfe/components/directory/nsDirectoryViewer.cpp
@@ -325,7 +325,7 @@ nsHTTPIndex::OnStopRequest(nsIRequest *request,
 
   nsresult rv;
 
-  nsXPIDLCString commentStr;
+  nsCString commentStr;
   mParser->GetComment(getter_Copies(commentStr));
 
   nsCOMPtr<nsIRDFLiteral> comment;
@@ -379,7 +379,7 @@ nsHTTPIndex::OnIndexAvailable(nsIRequest* aRequest, nsISupports *aContext,
   // we found the filename; construct a resource for its entry
   nsAutoCString entryuriC(baseStr);
 
-  nsXPIDLCString filename;
+  nsCString filename;
   nsresult rv = aIndex->GetLocation(getter_Copies(filename));
   if (NS_FAILED(rv)) return rv;
   entryuriC.Append(filename);