You need to update Firefox, like now!
You need to install updates on your browser at once, because there's an exploit in it that wants to steal your data and has been turning up on websites and causing havoc.
Right away, go to Help, then hit 'About Firefox', then press the 'Check for Updates' button, to ensure you've got the latest version of the browser.
In a blog, Mozilla say the exploit makes use of a weakness in Firefox's PDF viewer. The bug basically gets into your Windows computer and searches through your files looking for passwords from a host of popular FTP apps, as well as any text files with 'pass' or 'access' in the name. It will then, you suspect, send all that information to people who you really don't want to be having that sort of information.
Even if you're on a Mac and using Firefox, it'd be a good idea to do an update, as there's no good reason why the baddies aren't going after you too. So hurry up. Update your Firefox. Do it now!