If you want to sign up for newsletters and promotions sent out by various websites, you need to navigate to those websites in your browser and use the Sign Up feature that they offer. The process may vary somewhat between different sites.

For example, if you click on the "Subcribe and Save" button on cnet.com's homepage, you can enter your email address to sign up for their newsletter. You can't manage a list of email subscriptions in Firefox.

Some e-mail providers have a sorting option for incoming mail. If yours has this, be sure to keep an eye on the Junk / Spam folder for mails that you want to see. And to add them to the Safe Senders List.

How do I add mails to the safe senders list.

You have to contact your e-mail provider for more information.