Firefox does not have anything to do with your email or spam you may have received. Firefox is simply a browser that you use to access your email (Email is managed by people like AOL, Gmail, etc.). And if you received an email trying to sell you software, that came from a third party that is spamming you.

Shield Anti-virus sounds like it may be part of a family of rogue anti-viruses. http://en.wikipedia.org/wiki/Rogue_security_software. So, it sounds like you may have an infection on your machine if you installed this program. Also, if you provided your credit card to the anti-virus, the integrity of your credit card number is at stake.

Troubleshoot Firefox issues caused by malware provides some resources you can use to help eradicate this virus.

Thank you for your response. I gather that the e-mail itself may be connected with the "rogue antivirus" seller--in other words, the very people who send contaminated e-mail are also trying to sell their rogue antivirus to you. I say this because every time I attempted to get out of the contaminated e-mail and run Kapersky (which I did, eventually, to clear my computer of 18 different contaminants!), I'd get thrown back into the loop to purchase that $89.99 program. It's really awful to know that people are expending so much energy to commit crimes, like stealing your credit card number. It's sad that technology breeds criminals. Thanks for your help; I simply will remember to regard spam as spam and not open anything in that box to avoid this in the future, no matter how authentic it appears.

Basically, the scam artists that make the rouge antivirus sent you an email to try to get you to download it. You are most likely infected, and should follow the steps I gave in my last post to clean the infection off your PC

Trust me, I cleared it with Kapersky, which worked just fine. I learned my lesson about spam the hard way. No more for me, thanks.