I believe that the version of Netscape that you were using included the Netscape email client as part of it's suite.
Firefox does not.
Thunderbird, as some of the other folks have suggested, is a gret option for you to use in order to access your email.
You won't be able to add an icon from within Firefox, however, in order to have direct access to your email. In other words, your email application will be seperate from your browser application. (Firefox/Thunderbird)
It might be helpful to know what type of email account you are using. Is it an on-line account such as yahoo mail or google mail?
If it is then you can just add a "bookmark" to this email site an access your email account right from Firefox.
If you are using an ISP acount, such as Comcast or Earthlink, you can still do this BUT it will be much more convienient for you to set up an email client program such as Thunderbird or MS Outlook Express in order to access your email.
Hope that helps...?
Let me us know what else we can do to help you out with this...