diff --git a/extensions/firefox/content/PdfJs.jsm b/extensions/firefox/content/PdfJs.jsm
index 7cfbd2995..f36fa75a7 100644
--- a/extensions/firefox/content/PdfJs.jsm
+++ b/extensions/firefox/content/PdfJs.jsm
@@ -115,6 +115,7 @@ let PdfJs = {
     if (currentVersion >= VERSION) {
       return;
     }
+    // Make pdf.js the default pdf viewer on the first migration.
     if (currentVersion < 2) {
       this._becomeHandler();
     }