Could you provide a little more information about the email and about the add-ons? I'm not sure who is recommending them or where they are located online, both of which would have a bearing on whether you can trust the recommendations.

There is a Mozilla blog post that could be related to the email. I think I would trust the links in the blog post more than an email!

The Den > Dec. 4, 2013 > 13 Best Firefox Add-ons of 2013

By the way, if the warning you see is the pop-up in the attached, that is normal. This is a reminder that when you install add-ons from this or any other site, you need to check it out beforehand to make sure you trust it.