Firefox doesn't do email and doesn't have a preference setting for that. If you are talking about accessing "web mail" through Firefox, you need to read the support pages at your web mail service providers website to see how that's done, or look through the preference settings in your account. I would expect a setting like that to be under a category like Composing or Replies, but there might not even be a setting for that at your service provider. My web host, Domain Monger, has a setting for a feature like that in their web mail software, but Comcast doesn't - it is either on or off.