Sorry, that is not a Firefox support issue, Firefox doesn't do email. You may need to adjust your spam filters.

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, see this forum for support. http://www.mozillamessaging.com/en-US/support/
or this one
http://forums.mozillazine.org/viewforum.php?f=39

Make sure that you never post your email address on a public forum if you want to minimize the risk to receive spam messages. Also never respond to such unsolicited messages if the have a link to unsubscribe as that will only confirm that the email exists and is in use.

Further to the above, in July 2010 there were typically around 230-240 billion spam messages sent each day. By the end of December this had dropped to just over 40 billion messages per day. This downward trend appears to have stopped and in the first few weeks of January it has been increasing and is now at around 80 billion messages per day.

Source: http://www.symantec.com/business/security_response/landing/spam/index.jsp