Mozilla Firefox is a web browser, which provides a (far better) substitute for Internet Explorer. If you normally send your mail by logging onto your provider's website (in Internet Explorer) you can do exactly the same with Firefox.
However Firefox is not an email client. If you normally send your mail via a dedicated client (such as Outlook, Outlook Express or Windows Mail) you'll need to keep doing so unless you also download and install Firefox's sister program, Thunderbird (which is an email client), and use that instead.
Chris