You posted in the Firefox support fora queue. Firefox doesn't do email, it's strictly a web browser.

If you are using Firefox to access your mail, you are using "web-mail" such as Yahoo!Mail or Gmail. You need to seek support from your service provider or a forum for that service.

If your problem is with Thunderbird, let us know and we can move this thread to the Thunderbird support queue.

https://support.mozilla.org/en-US/kb/problems-with-email-how-to-find-help

Would the problem have anything to do with Add-ons or Extensions?

Hi   !
You could try and ask that retailer's customer service why they quit sending you emails  ..........

I signed up on my computer to receive them, but somehow I am not getting them. This started happening about 3 weeks ago. They do not understand why I am not receiving them either.

ChaCha said

They do not understand why I am not receiving them either.

So they're saying that they   did   send them   ......
In that case you really need to contact your webmail provider.

Such e-mail is always sent to the address on file with the website.

Check your user profile at the site.