Firefox 12 was the last version for Windows 2000. You can get that version from the FTP server, which hosts all previous releases of Firefox. Because older releases contain publicly disclosed security vulnerabilities, unfortunately you probably need to consider alternative browsers for Windows 2000.

ftp://ftp.mozilla.org/pub/firefox/releases/12.0/win32/en-US/

(If you prefer a language other than U.S. English, navigate up a level to choose your language.)

You may want to read this support thread about using newer versions (than 12.0) of Firefox with Windows 2000.

http://forums.mozillazine.org/viewtopic.php?f=23&t=2482475