Firefox no longer works with Windows 2000
Firefox 3.6.28 and Firefox 12 are the last versions of Firefox that will work with Windows 2000. These outdated versions are not updated with security and stability fixes. To keep your computer and personal information safe, we recommend that you upgrade Windows and then install the latest Firefox, if possible.
Since Windows 2000 is no longer supported by Microsoft it's recommended that you upgrade to Windows 7 (or above) if possible. This is the most secure option and will allow you to run the latest and greatest Firefox.