diff --git a/pkgs/applications/networking/browsers/firefox-bin/default.nix b/pkgs/applications/networking/browsers/firefox-bin/default.nix
index b768bfd61115..219f208af71e 100644
--- a/pkgs/applications/networking/browsers/firefox-bin/default.nix
+++ b/pkgs/applications/networking/browsers/firefox-bin/default.nix
@@ -236,7 +236,7 @@ stdenv.mkDerivation {
         }
         EOF
 
-        cat $tmpfile > ${if isBeta then "beta_" else ""}sources.nix
+        mv $tmpfile ${if isBeta then "beta_" else ""}sources.nix
 
         popd
       '';