Edit: if you use the FireFox web browser, then disregard the thread title and just get whatever the latest version is. You can get it from http://www.getfirefox.com. Like other web browsers, it is good to keep it up-to-date so you're not running around with known security vulnerabilities. Also check for updated versions of your preferred media-player softwares, since they're sometimes exploited too. The bad guys mean business (literally) so minimize your attack surface
Windows users, if you don't have any antivirus software, or it's some ancient relic from years ago, try a free-for-home-users antivirus like AntiVir: http://www.free-av.com Make sure to turn on all the options (heuristics, compressed-file / archive scanning).
"We're identifying this as a critical release, and we're strongly recommending that everyone update as soon as possible," Mike Schroepfer, vice president of engineering at Mozilla, said in a statement.
Well, there you have it. Update to version 220.127.116.11 to fix several Critical-level security vulnerabilties. http://www.getfirefox.com is one place you can download from.
Regardless of whether you use IE or not, you should also make sure your system's got its Automagic Updates turned on in Control Panel, like this:
Also, if you visit risky sites (and you know what I mean) then set up a Limited user account, and/or avoid risky sites. Web browsers are far safer if you run them as a Limited user, rather than a Computer Administrator. http://www.mechbgon.com/build/Limited.html <-- info. Scream if you need help