You knew it had to happen. Malware for Firefox. It happens all the time with IE (so much so that my 17-year-old niece needs a fresh install of Windows every 3 months), but Firefox has been a little less prone — though not imune — to malware. See the BitDefender blog post and the Infoworld article which has a bit more detail.
Now Firefox 3 does contain Malware protection, but apparently this plugin is delivered via other system compromises to the filesystem, and apparently Firefox doesn’t question already installed plugins.