diff --git a/tests/firefox.nix b/tests/firefox.nix
index e2f10fa97ab4..6bc202de84bf 100644
--- a/tests/firefox.nix
+++ b/tests/firefox.nix
@@ -14,7 +14,7 @@
       $machine->waitForX;
       $machine->execute("firefox file://${pkgs.valgrind}/share/doc/valgrind/html/index.html &");
       $machine->waitForWindow(/Valgrind.*Shiretoko/);
-      sleep 30; # wait until Firefox has finished loading the page
+      sleep 40; # wait until Firefox has finished loading the page
       $machine->screenshot("screen");
     '';