diff --git a/mailnews/base/util/nsMsgUtils.cpp b/mailnews/base/util/nsMsgUtils.cpp
index 9409fd5586..a25f277c44 100644
--- a/mailnews/base/util/nsMsgUtils.cpp
+++ b/mailnews/base/util/nsMsgUtils.cpp
@@ -2105,10 +2105,10 @@ NS_MSG_BASE nsMsgKey msgKeyFromInt(uint64_t aValue)
 }
 
 // Helper function to extract a query qualifier.
-nsAutoCString MsgExtractQueryPart(nsAutoCString spec, const char* queryToExtract)
+nsCString MsgExtractQueryPart(const nsACString& spec, const char* queryToExtract)
 {
-  nsAutoCString queryPart;
-  int32_t queryIndex = spec.Find(queryToExtract);
+  nsCString queryPart;
+  int32_t queryIndex = nsPromiseFlatCString(spec).Find(queryToExtract);
   if (queryIndex == kNotFound)
     return queryPart;
 
diff --git a/mailnews/base/util/nsMsgUtils.h b/mailnews/base/util/nsMsgUtils.h
index 1920ba780d..455897d129 100644
--- a/mailnews/base/util/nsMsgUtils.h
+++ b/mailnews/base/util/nsMsgUtils.h
@@ -372,7 +372,7 @@ NS_MSG_BASE nsMsgKey msgKeyFromInt(uint64_t aValue);
 /**
  * Helper function to extract query part from URL spec.
  */
-nsAutoCString MsgExtractQueryPart(nsAutoCString spec, const char* queryToExtract);
+nsCString MsgExtractQueryPart(const nsACString& spec, const char* queryToExtract);
 
 /**
  * Helper macro for defining getter/setters. Ported from nsISupportsObsolete.h
diff --git a/mailnews/imap/src/nsImapProtocol.cpp b/mailnews/imap/src/nsImapProtocol.cpp
index d238788478..ae8faa281e 100644
--- a/mailnews/imap/src/nsImapProtocol.cpp
+++ b/mailnews/imap/src/nsImapProtocol.cpp
@@ -9361,7 +9361,7 @@ nsresult nsImapMockChannel::OpenCacheEntry()
   // ?section=2?part=1.2&filename=A01.JPG&type=image/jpeg&filename=A01.JPG
   // ?part=1.2&type=image/jpeg&filename=IMG_C0030.jpg
   // ?header=quotebody&part=1.2&filename=lijbmghmkilicioj.png
-  nsAutoCString partQuery = MsgExtractQueryPart(path, "?part=");
+  nsCString partQuery = MsgExtractQueryPart(path, "?part=");
   if (partQuery.IsEmpty()) {
     partQuery = MsgExtractQueryPart(path, "&part=");
     if (!partQuery.IsEmpty()) {
@@ -9369,7 +9369,7 @@ nsresult nsImapMockChannel::OpenCacheEntry()
       partQuery.SetCharAt('?', 0);
     }
   }
-  nsAutoCString filenameQuery = MsgExtractQueryPart(path, "&filename=");
+  nsCString filenameQuery = MsgExtractQueryPart(path, "&filename=");
 
   // Truncate path at either /; or ?
   int32_t ind = path.FindChar('?');
diff --git a/mailnews/jsaccount/src/JaUrl.cpp b/mailnews/jsaccount/src/JaUrl.cpp
index f103460c66..21b1163fea 100644
--- a/mailnews/jsaccount/src/JaUrl.cpp
+++ b/mailnews/jsaccount/src/JaUrl.cpp
@@ -108,7 +108,7 @@ NS_IMETHODIMP JaBaseCppUrl::GetPrincipalSpec(nsACString& aPrincipalSpec)
   nsAutoCString spec;
   mailnewsURL->GetSpecIgnoringRef(spec);
 
-  nsAutoCString queryPart = MsgExtractQueryPart(spec, "number=");
+  nsCString queryPart = MsgExtractQueryPart(spec, "number=");
 
   // Strip any query part beginning with ? or /;
   int32_t ind = spec.Find("/;");