--- a/uriloader/exthandler/win/nsMIMEInfoWin.cpp
+++ b/uriloader/exthandler/win/nsMIMEInfoWin.cpp
@@ -508,17 +508,17 @@ void nsMIMEInfoWin::ProcessPath(nsCOMPtr
 {
   nsAutoString lower(appFilesystemCommand);
   ToLowerCase(lower);
 
   // Don't include firefox.exe in the list
   WCHAR exe[MAX_PATH+1];
   uint32_t len = GetModuleFileNameW(nullptr, exe, MAX_PATH);
   if (len < MAX_PATH && len != 0) {
-    uint32_t index = lower.Find(exe);
+    int32_t index = lower.Find(exe);
     if (index != -1)
       return;
   }
 
   nsCOMPtr<nsILocalHandlerApp> aApp;
   if (!GetLocalHandlerApp(appFilesystemCommand, aApp))
     return;