Mozilla released Firefox version 3.6.7 today to combat a number of security risks. With the volume of infected computers I've dealt with lately in mind, I urge you to install this update as soon as possible.
If you have a recent version of Firefox (3.6.*) you can update by opening Firefox and allowing the update to automatically download, then you agree to install. Or you can click Help - Check for Updates and follow the prompts.
If you have an earlier version of Firefox, or don't have it installed yet, download and install the complete version at http://www.mozilla.com/en-US/
Details on the fix can be found here.