If you are talking about executable viruses which can be deployed on your system by clicking on a link to something then yes, Linux is probably more secure than Windows because those viruses are written to exploit vulnerabilities and poor security design in Windows.
However, if you are talking about security vulnerabilities specific to Firefox like memory corruption and privilege escalation, then those threats exist regardless of what platform you're running Firefox on. It's up to Firefox to fix those problems and it's up to you to stay updated with the most current releases.
The theft of credit card info is usually not the fault of your browser. Either you gave that information to an insecure service or an attacker extracted it via their own attack. E-bay had a problem with this not too long ago where attackers sent a phishing e-mail (one where the sender masquerades themselves as a trusted source, E-bay in this case) and asks you to confirm your account by entering your password or credit card number.
If you're vigilant when browsing the Internet and you use a pop-up blocker, I don't think you'll encounter too many problems. Just be sure you trust the source when anybody tries to link you to a site. If you'd like to keep up on the latest Mozilla vulnerabilities, you can bookmark this page. |