Selenium IDE command "execute script" not working

Question

I'm a beginner with Selenium IDE. I want to use the "execute script" command. I created a two-line script to try it out but I'm not seeing the results I expect.

In Selenium IDE I created these two commands:

open http://www.duckduckgo.com

execute script alert("Hello Duck")

I run the test and the browser window (Firefox) opens to the specified URL and then the 'execute script' command turns green to indicate it was performed, but no alert dialog appears.

Am I doing something wrong?

Note that I tried this in Chromium with the Selenium IDE extension and it works fine.