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". You need to seek support from your service provider or a forum for that service.

If your problem is with Mozilla Thunderbird, let us know and we can move this thread to the Thunderbird queue. This question currently is in the Firefox queue for answers.

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