Henrys repeatedly posts that answer whenever this question comes up (which is quite often!). Firefox is a web browser, not an e-mail client and will do absolutely nothing to reduce spam. (I'm not knocking Firefox. It's far superior to Internet Explorer but it has nothing to do with POP3/SMTP e-mail).
You can't prevent spam from arriving on the server of your e-mail provider (other than by using multiple accounts, as below). The only thing you can do is to delete it from the server before it's downloaded to your Inbox. The most frequently recommended tool for this job is MailWasher:
http://www.mailwasher.net/
I don't use MailWasher myself, as I prefer a different approach. Rather than just have one e-mail address, I use different addresses for different purposes. For example, I've got a current e-mail address which I use on AB whenever I want someone to contact me directly. It's not used for any other purpose. I've never received any spam to this address but, if I do, I'll just change the address. (Most ABers who need to contact me only need to do so for a very limited period, so it doesn't matter that I no longer use an old address). I also create separate e-mail addresses for every discussion group that I contribute to. Once again, if I start to get spam, I simply change to a new address. My 'personal' e-mail address (i.e. the one assigned by my ISP) is only made known to a very small number of people. If you need to set up new e-mail accounts (which can be used with Outlook Express), the best free service is based in Egypt (Gawab.com):
http://www.gawab.com/webfront/main.php
Chris