It seems that some Firefox users don't use or know about add-ons. I think that including suggestions of popular add-ons in the welcome page will help. They can be put into groups depending on their functionality.
This would be good for users who don't typically look at extensions as it would allow them to see some of the great extensions that are available.
I believe add ons like the containers are crucial to helping Firefox stand out from other browsers, and are key to retaining users, once they start using them. Having this one time pop up will expose new users to these and then potentially keep them from switching away in future.