Firefox doesn't do email, it's 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, see this forum for support. http://www.mozillamessaging.com/en-US/support/
or this one
http://forums.mozillazine.org/viewforum.php?f=39

This is happening to me too with my Yahoo mail. It doesnt happen with Internet Explorer, and it doesnt happen with my Firefox installation on my home computer, just my work computer, so I dont think its a Yahoo problem.

Any suggestions would be greatly appreciated.