As Microsoft announced IE 10 preview program 1, Mozilla has also announced development of Firefox 5 and Firefox 6. These version should only be used for development purpose. It is available for download at Mozilla Public FTP.
Download FireFox 5:
http://ftp.mozilla.org/pub/mozilla.org/firefox/nightly/2011-04-13-09-mozilla-aurora/
Download FireFox 6 (Alfa):
http://ftp.mozilla.org/pub/mozilla.org/firefox/nightly/latest-tracemonkey/