That would be a question for whatever service that you use for email. 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 when using Thunderbird, let us know and we can move this thread to the Thunderbird support queue. But the "size" limitation would probably be set by the email portal. https://support.mozilla.org/en-US/kb/problems-with-email-how-to-find-help