Module 10 - In Salesforce, no alert when deleting a lead

In Module 10 - Selenium IDE, we need to handle JS alerts. The problem is that the alert is not shown in Firefox (when I perform action manually).

The use case in the video deletes a lead in Salesforce and a JS alert is shown.

Here is what I see in Firefox 47.0:
When I select 'Tom Hanks' lead, there is a Delete button - but nothing happens when I click on it. Other buttons (like Edit, Convert work). Also this delete button works in Microsoft Edge and the alert is also displayed.

When I display a list of leads,
I can use a delete link next to a lead to delete a specific lead
Or
Use delete link at top of list to delete all selected leads
In both cases, there is no alert before the delete.

In mass-delete leads, there is a delete button. But clicking this just deletes the leads that satisfy query, and no alert is displayed before the delete (pl. see screen shot).
[attachment=0:3m7qyhln]no js alert.png[/attachment:3m7qyhln]
Can you pl. help - I would like to handle the alert pop-up in my Selenium IDE. Thank you