What error are you receiving?

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, see this forum for support. https://support.mozillamessaging.com/.../home
or this one
http://forums.mozillazine.org/viewforum.php?f=39

No error code or messsage. Just a pop-up box that states to the effect "unable to send you message. try later" Been trying for over a week now, without success.

Hello,

In order to better assist you with your issue please provide us with a screenshot. If you need help to create a screenshot, please see How do I create a screenshot of my problem?

Once you've done this, attach the saved screenshot file to your forum post by clicking the Browse... button below the Post your reply box. This will help us to visualize the problem.

Thank you!