diff --git a/extensions/firefox/content/PdfJs.jsm b/extensions/firefox/content/PdfJs.jsm
index 3c790c17b..5522312d6 100644
--- a/extensions/firefox/content/PdfJs.jsm
+++ b/extensions/firefox/content/PdfJs.jsm
@@ -224,7 +224,7 @@ let PdfJs = {
       }
       let mimeTypes = tag.getMimeTypes();
       return mimeTypes.some(function(mimeType) {
-        return mimeType.type === PDF_CONTENT_TYPE;
+        return mimeType === PDF_CONTENT_TYPE;
       });
     });