On the usual channels (like http://www.mozilla.com/en-US/firefox/channel/ ) you can only download the 32bit version of the latest Firefox release.

But I stumbled over a public Mozilla FTP where they hold all releases, including the 64bit one.

Link: ftp://ftp.mozilla.org/pub/mozilla.org/firefox/releases/

This way all the installed plug-ins on your 64bit *nix system will work out of the box (including Adobe Flash).