As Microsoft released platform preview for IE10, Mozilla seems to be on top gear to push their development process to get their 5th and 6th major version towards release.
FireFox 5 and FireFox 6 (Alfa) is now available via 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/
Please do keep in mind, these builds are internal test builds and expected to be buggy hence if you plan to use them, use them in test purpose only.