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

Some e-mail providers have the option to sort incoming mail. Contact your e-mail provider for help.

Sorting the emails is not really the issue, although it is part of the problem, Regardless of whether the spam from invalid email address is sent to my inbox (which usually happens) or junk folder (which is what I prefer), I want to be able to BLOCK future emails from the invalid email address. Firefox and Microsoft do not permit me to block an invalid email address. Why"?

Firefox, and even Thunderbird, have no way to do this. If your provider has the option, sort all incoming to your junk mail folder but have known addresses sent to the inbox.

Since Firefox and Thunderbird have the ability to recognize invalid email addresses when I try to block the emails, why (and how) do they permit an invalid email address to send me the spam in the first place? Why don't Firefox and Thunderbird automatically return to sender (or even delete) emails from invalid addresses? Why would anyone possibly want to receive an email from an invalid email address? It wouldn't be possible to reply even if one was silly enough to try. Obviously an invalid address must be from a spammer, so why assist them by delivering their junk emails?

moving to the Thunderbird support queue